Adaptive cardiac resynchronization therapy

ABSTRACT

Cardiac resynchronization therapy (CRT) delivered to a heart of a patient may be adjusted based on detection of a surrogate indication of the intrinsic atrioventricular conduction of the heart. In some examples, the surrogate indication is determined to be a sense event of the first depolarizing ventricle of the heart within a predetermined period of time following the delivery of a fusion pacing stimulus to the later depolarizing ventricle. In some examples, the CRT is switched from a fusion pacing configuration to a biventricular pacing configuration if the surrogate indication is not detected, and the CRT is maintained in a fusion pacing configuration if the surrogate indication is detected.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/361,635, filed Jan. 30, 2012 entitled “ADAPTIVE CARDIACRESYNCHRONIZATION THERAPY”, herein incorporated by reference in itsentirety.

TECHNICAL FIELD

The disclosure relates to implantable medical devices, and, moreparticularly, to cardiac therapy delivery by implantable medicaldevices.

BACKGROUND

Some types of implantable medical devices, such as cardiac pacemakers orimplantable cardioverter defibrillators, provide therapeutic electricalstimulation to a heart of a patient via electrodes of one or moreimplantable leads. The therapeutic electrical stimulation may bedelivered to the heart in the form of pulses or shocks for pacing,cardioversion or defibrillation. In some cases, an implantable medicaldevice may sense intrinsic depolarizations of the heart, and control thedelivery of therapeutic stimulation to the heart based on the sensing.

Cardiac resynchronization therapy is one type of therapy delivered by animplantable medical device. Cardiac resynchronization therapy may helpenhance cardiac output by resynchronizing the electromechanical activityof the ventricles of the heart. Ventricular desynchrony may occur inpatients that suffer from congestive heart failure (CHF).

SUMMARY

In general, the disclosure is directed to adaptive cardiacresynchronization (CRT) pacing therapy in which the type of pacingtherapy (referred to herein as a “pacing configuration”) is adjustedbased on whether a surrogate indication of the presence of intrinsicatrioventricular conduction (also referred to as “intrinsic AVconduction”) from an atrium of the heart to a first depolarizingventricle is detected. The types of pacing therapies of the adaptive CRTmay include, for example, fusion pacing and biventricular pacing, whichmay be delivered to a patient at different times. In some examples, thesurrogate indication of intrinsic AV conduction includes a detection ofventricular activation of the first depolarizing ventricle (V1_(S)) ofthe heart within a predefined time window immediately following thedelivery of a fusion pacing stimulus to the later depolarizing ventricle(V2_(P)). In some examples, the detected ventricular activation (V1_(S))is confirmed to not be attributable to the fusion pacing stimulus(V2_(P)) prior to characterizing the ventricular activation (V1_(S)) asthe surrogate indication of intrinsic AV conduction. A medical devicemay determine whether the surrogate indication of intrinsic AVconduction is detected on a beat-by-beat basis in some examples, and ona less frequent basis in other examples.

In some examples, the time interval between an atrial pace or senseevent (A_(P/S)) and the an event (indicative of depolarization) sensedin the first depolarizing ventricle (V1_(S)) (within the predeterminedwindow of time following the fusion pacing stimulus) may be periodicallyused to adjust the timing of the pacing stimuli, e.g., delivered to thelater depolarizing ventricle (V2_(P)). For example, a fusion pacinginterval may be adjusted based on the A_(P/S)−V1_(S) interval.

In one example, the disclosure is directed to a system comprising anelectrical stimulation module configured to deliver cardiacresynchronization pacing therapy to a heart of a patient, a sensingmodule, and a processor configured to control the electrical stimulationmodule to deliver a pacing stimulus to a first ventricle of a heart of apatient, and determine whether a surrogate indication of intrinsicconduction of the heart of the patient is detected after the electricalstimulation module delivers the pacing stimulus to the first ventricle.The processor is configured to determine whether the surrogateindication of the intrinsic conduction is detected by at leastdetermining whether the sensing module detected activation of a secondventricle of the heart within a predetermined window of time immediatelyfollowing delivery of the pacing stimulus to the first ventricle by theelectrical stimulation module. In addition, the processor is configuredto control the cardiac resynchronization therapy delivered by theelectrical stimulation module to the patient based on whether thesurrogate indication of intrinsic conduction is detected after theelectrical stimulation module delivers the pacing stimulus to the firstventricle.

In another aspect, the disclosure is directed to a method comprisingcontrolling an electrical stimulation module to deliver a pacingstimulus to a first ventricle of a heart of a patient, and, after themedical device delivers the pacing stimulus to the first ventricle,determining whether a surrogate indication of intrinsic conduction ofthe heart of the patient is detected. Determining whether the surrogateindication of intrinsic conduction is detected comprises detectingactivation of a second ventricle of the heart within a predeterminedwindow of time immediately following delivery of the pacing stimulus tothe first ventricle. The method further comprises controlling cardiacresynchronization therapy delivered by the electrical stimulation moduleto the patient based on whether the surrogate indication of intrinsicconduction is detected.

In another aspect, the disclosure is directed to a system comprisingmeans for delivering cardiac resynchronization therapy to a heart of apatient, and means for determining whether a surrogate indication ofintrinsic conduction of the heart of the patient is detected after themeans for delivering cardiac resynchronization therapy delivers a pacingstimulus to a first ventricle of the heart. The means for determiningwhether the surrogate indication of intrinsic conduction is detectedcomprises means for detecting activation of a second ventricle of theheart within a predetermined window of time immediately followingdelivery of the pacing stimulus to the first ventricle. The systemfurther comprises means for controlling cardiac resynchronizationtherapy delivered by the means for delivering cardiac resynchronizationtherapy based on whether the surrogate indication of intrinsicconduction is detected.

In another aspect, the disclosure is directed to a computer-readablemedium containing instructions. The instructions, when executed by aprocessor, cause the processor to control an electrical stimulationmodule to deliver a pacing stimulus to a first ventricle of a heart of apatient, and, after the medical device delivers the pacing stimulus tothe first ventricle, determine whether a surrogate indication ofintrinsic conduction of the heart of the patient is detected. Theinstructions cause the processor to determine whether the surrogateindication of the intrinsic conduction is detected by at least detectingactivation of a second ventricle of the heart within a predeterminedwindow of time immediately following delivery of the pacing stimulus tothe first ventricle. The instructions also cause the processor tocontrol cardiac resynchronization therapy delivered by the electricalstimulation module to the patient based on whether the surrogateindication of intrinsic conduction of the heart of the patient isdetected.

In another aspect, the disclosure is directed to a computer-readablestorage medium comprising computer-readable instructions for executionby a processor. The instructions cause a programmable processor toperform any whole or part of the techniques described herein. Theinstructions may be, for example, software instructions, such as thoseused to define a software or computer program. The computer-readablemedium may be a computer-readable storage medium such as a storagedevice (e.g., a disk drive, or an optical drive), memory (e.g., a Flashmemory, read only memory (ROM), or random access memory (RAM)) or anyother type of volatile or non-volatile memory that stores instructions(e.g., in the form of a computer program or other executable) to cause aprogrammable processor to perform the techniques described herein. Insome examples, the computer-readable medium is an article of manufactureand is non-transitory.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an example therapy system.

FIG. 2 is a conceptual diagram illustrating the medical device and leadsof the therapy system of FIG. 1 in greater detail.

FIG. 3 is a functional block diagram of an example implantable medicaldevice that delivers stimulation to a heart of a patient.

FIG. 4 is a functional block diagram of an example medical deviceprogrammer.

FIGS. 5 and 6 are flow diagrams of example techniques for providingadaptive cardiac resynchronization therapy based on a surrogateindication of the presence of intrinsic AV conduction.

FIGS. 7-9 are flow diagrams of example techniques for determiningwhether the detection of a sense event in the first depolarizingventricle of the patient within a predetermined amount of time followingthe delivery of a fusion pacing stimulus to the later depolarizingventricle is a surrogate indication of intrinsic AV conduction.

FIG. 10 is a flow diagram of an example technique for determiningwhether a pattern in A_(P/S)−V1_(S) intervals over time is indicative ofloss of intrinsic AV conduction.

FIG. 11 is a flow diagram of another example technique for determiningthe detection of a sense event in the first depolarizing ventricle ofthe patient within a predetermined amount of time following the deliveryof a fusion pacing stimulus to the later depolarizing ventricle is asurrogate indication of intrinsic AV conduction.

FIG. 12 is a table that compares example a peak R-wave amplitude of acardiac electrogram when the R-wave is known to be attributable to apacing pulse to a peak R-wave amplitude of a cardiac electrogram whenthe R-wave is known to be attributable to intrinsic conduction.

FIG. 13 is a flow diagram of an example technique that may beimplemented to adjust a fusion pacing interval based on one or moreA_(P/S)−V1_(S) intervals.

FIG. 14 is a block diagram illustrating an example system that includesan external device, such as a server, and one or more computing devicesthat are coupled to the IMD and programmer shown in FIG. 1 via anetwork.

DETAILED DESCRIPTION

Devices, systems, and techniques for providing adaptive cardiacresynchronization (CRT) pacing are described herein. A medical deviceconfigured to provide adaptive CRT is configured to deliver pacingstimuli (e.g., pacing pulses) to the heart of the patient toresynchronize the electromechanical activity of the ventricles of theheart of a patient. The timing of the pacing stimuli may be controlled,e.g., by a processor, based on a pacing interval, which may be, forexample, the duration of time following an atrial pace or sense event atwhich the pacing stimulus is delivered to the heart. Resynchronizationof the electromechanical activity of the ventricles of the heart withthe aid of CRT may be useful for patients with heart failure, orintraventricular or interventricular conduction delays (e.g., left orright bundle branch block).

In some proposed medical devices configured to provide adaptive CRT, apacing configuration, e.g., a fusion pacing configuration (which mayalso be referred to as uni-ventricular fusion pacing) or a biventricularpacing configuration, and timing of the pacing stimuli based on periodicintrinsic conduction measurements may be periodically adjusted toachieve more efficient physiologic pacing and to improve hemodynamics ofthe patient. Fusion pacing and biventricular pacing are described infurther detail below. While the pacing stimuli may be pacing pulses orcontinuous time signals, the pacing stimuli are primarily referred toherein as pacing pulses for ease of description.

Fusion-based cardiac resynchronization therapy (also referred to hereinas fusion pacing) may be useful for restoring a depolarization sequenceof a heart of a patient, which may be irregular due to ventriculardysfunction, in patients with preserved intrinsic atrial-ventricular(AV) conduction. In a fusion pacing configuration, a medical devicedelivers one or more fusion pacing pulses to a later-contractingventricle (V2) in order to pre-excite the V2 and synchronize thedepolarization of the V2 with the depolarization of the earliercontracting ventricle (V1). The ventricular activation of the V2 may“fuse” (or “merge”) with the ventricular activation of the V1 that isattributable to intrinsic conduction of the heart. In this way, theintrinsic and pacing-induced excitation wave fronts may fuse togethersuch that the depolarization of the V2 is resynchronized with thedepolarization of the V1.

The medical device may be configured to deliver the fusion pacing pulseto the V2 according to a fusion pacing interval, which indicates thetime relative to an atrial pace or sense event at which the fusionpacing pulse should be delivered to the V2. An atrial sense event maybe, for example, a P wave of a sensed electrical cardiac signal and anatrial pacing event may be, for example, the time at which a stimulus isdelivered to the atrium.

In some examples, the right ventricle (RV) may be the V1 and the leftventricle (LV) may be the V2. In other examples, the LV may be the V1while the RV may be the V2. While the disclosure primarily refers toexamples in which the first depolarizing ventricle V1 is the RV and thelater depolarizing ventricle V2 is the LV, the devices, systems,techniques described herein for providing CRT may also apply to examplesin which the first depolarizing ventricle V1 is the LV and the laterdepolarizing ventricle V2 is the RV.

In some fusion pacing techniques, a pacing pulse to the V2 (V2_(P)) isdelivered upon expiration of a fusion pacing interval that is determinedbased on the intrinsic depolarization of the V1, which may be indicatedby a sensing of ventricular activation (V1_(S)). Ventricular activationmay be indicated by, for example, an R-wave of a sensed electricalcardiac signal. An example of a fusion pacing technique that times thedelivery of the V2 pacing pulse (V2_(P)) to the intrinsic depolarizationof the V1 is described in U.S. Pat. No. 7,181,284 to Burnes et al.,which is entitled, “APPARATUS AND METHODS OF ENERGY EFFICIENT,ATRIAL-BASED BI-VENTRICULAR FUSION-PACING,” and issued on Feb. 20, 2007.U.S. Pat. No. 7,181,284 to Burnes et al. is incorporated herein byreference in its entirety.

In one example disclosed by U.S. Pat. No. 7,181,284 to Burnes et al., apacing pulse to the V2 (V2_(P)) is delivered a predetermined period oftime following an atrial pace or sense event (A_(P/S)), where thepredetermined period of time is substantially equal to the duration oftime between the atrial pace or sense event (A_(P/S)) and a V1 sensingevent (V1_(S)) of at least one prior cardiac cycle decremented by aduration of time referred to as the pre-excitation interval (PEI). Thus,one example equation that may be used to determine a fusion pacinginterval (A_(P/S)−V2_(P)):

A _(P/S) −V2_(P)=(A _(P/S) −V1_(S))−PEI  Equation (1)

A cardiac cycle may include, for example, the time between the beginningof one heart beat to the next heartbeat. The duration of time betweenthe atrial pace or sense event (A_(P/S)) and a V1 sensing event (V1_(S))may be, for example, a measurement of intrinsic AV conduction time froman atrium to the first contracting ventricle of the heart of thepatient. The PEI may indicate the amount of time with which a V2 pacingpulse precedes a V1 sensing event in order to achieve the fusing of theelectromechanical performance of the V1 and V2. That is, the PEI mayindicate the amount of time from the delivery of the V2 pacing pulsethat is required to pre-excite the V2, such that the electromechanicalperformance of V1 and V2 merge into a fusion event. In some examples,the PEI is automatically determined by a medical device delivering thepacing therapy, e.g., based on determined intrinsic conduction times,while in other examples, the PEI may be predetermined by a clinician. Insome examples, the PEI is a programmed value (e.g., about onemillisecond (ms) to about 250 ms or more, such as about 100 ms to about200 ms, or about 10 ms to about 40 ms) or is an adaptive value, such asabout 10% of a measured intrinsic A−V2 conduction delay or measuredintrinsic A−A cycle length.

The magnitude of the PEI may differ based on various factors, such asthe heart rate of the patient, a dynamic physiologic conduction statusof the heart of the patient, which may change based upon thephysiological condition of the patient (e.g., ischemia status,myocardial infarction status, and so forth), as well as factors relatedto the therapy system, such as the location of sensing electrodes of theleads of the therapy system, the location of the pacing electrodes ofthe therapy system, and internal circuitry processing delays of themedical device.

Another technique for determining the timing of the delivery of a pacingpulse to a later depolarizing ventricle (V2) (which is sometimes alsoreferred to as a “fusion pacing interval”) is described in U.S. PatentApplication Publication No. 2010/0198291 by Sambelashvili et al., whichis incorporated herein by reference in its entirety. In some examplesdescribed by U.S. Patent Application Publication No. 2010/0198291 bySambelashvili et al., the timing of the delivery of a pacing pulse isbased on a depolarization of the V2 in at least one prior cardiac cycle.The depolarization of the V2 may be detected by sensing an event in theV2 (V2_(S)), such as an R-wave of an electrical cardiac signal. The V2pacing pulse (V2_(P)) is timed such that an evoked depolarization of theV2 is effected in fusion with the intrinsic depolarization of the firstdepolarizing ventricle (V1), resulting in a ventricularresynchronization. In this way, the V2 pacing pulse (V2_(P)) maypre-excite the conduction delayed V2 and help fuse the activation of theV2 with the activation of the V1 from intrinsic conduction. The intervalof time between the V2 pacing pulse (V2_(P)) and the V2 sensing event(V2_(S)) of the same cardiac cycle may be referred to as the adjustedPEI.

In some examples disclosed by U.S. Patent Application Publication No.2010/0198291 by Sambelashvili et al., the predetermined period of timeat which an IMD delivers the V2 pacing pulse (V2_(P)) following anatrial pace or sense event (A_(P/S)) is substantially equal to theduration of time between an atrial event (sensed or paced) (A_(P/S)) anda V2 sensing event (V2_(S)) of at least one prior cardiac cycledecremented by a duration of time referred to as an adjusted PEI. Thatis, in some examples, the adjusted PEI indicates the desired interval oftime between the delivery of the V2 pacing pulse (V2_(P)) and the V2sensing event (V2_(S)) of the same cardiac cycle. One example equationthat may be used to determine the timing of a fusion pacing pulse usinga technique described by U.S. Patent Application Publication No.2010/0198291 by Sambelashvili et al. is:

A _(P/S) −V2_(P)delay=(A _(P/S) −V2_(S))−adjusted PEI  Equation (2)

The duration of time between an atrial pace or sense event (A_(P/S)) anda V2 sensing event (V2_(S)) may be referred to as an atrioventricular(AV) delay. The adjusted PEI may indicate an interval of time prior to aV2 sensing event (V2_(S)) at which it may be desirable to deliver the V2pacing pulse (V2_(P)) in order to pre-excite the V2 and merge theelectromechanical performance of V2 and V1 into a fusion event. In someexamples described by U.S. Patent Application Publication No.2010/0198291 by Sambelashvili et al., an adjusted PEI is a linearfunction that is based on V1 sensing event (V1_(S)) and a V2 sensingevent (V2_(S)) of the same cardiac cycle, based on the time between anatrial pace or sense event (A_(P/S)) and a V2 sensing event, or anycombination thereof.

As an example, adjusted PEI may be determined as follows:

Adjusted PEI=a(V1_(S) −V2_(S))+b  Equation (3)

According to U.S. Patent Application Publication No. 2010/0198291 bySambelashvili et al., in Equation (3), the coefficients “a” and “b” maybe fixed, empiric coefficients that are selected by a clinician ordetermined based on an adjusted PEI value selected by a clinician. Insome examples, the coefficient “a” in Equation (3) may be about 1 andthe coefficient “b” may be substantially equal to the PEI. In this case,the adjusted PEI is substantially equal to a time interval between a V1sensing event (V1_(S)) and a V2 sensing event (V2_(S)) of the samecardiac cycle, incremented by the PEI. As a result, the A_(P/S)−V2_(P)delay for timing the delivery of a V2 pacing pulse may be determined asfollows

A _(P/S) −V2_(P)delay=(A _(P/S) −V2_(S))−[(V1_(S)−V2_(S))+PEI)]  Equation (4)

Other values for the “a” and “b” coefficients in Equation (2) may beselected. In addition, other types of fusion pacing configurations mayalso be used in accordance with the techniques described herein. Forexample, other fusion pacing intervals described by U.S. Pat. No.7,181,284 to Burnes et al. and U.S. Patent Application Publication No.2010/0198291 by Sambelashvili et al. can also be used to control fusionpacing in accordance with techniques described herein. An example of CRTis described in U.S. Pat. No. 6,871,096 to Hill, which is entitled“SYSTEM AND METHOD FOR BI-VENTRICULAR FUSION PACING” and is incorporatedherein by reference in its entirety.

In contrast to fusion pacing, in a biventricular pacing configuration,the medical device may deliver pacing pulses to both the LV and the RVin order to resynchronize the contraction of the LV and RV. In abiventricular pacing configuration, a medical device may deliverstimulation to coordinate contraction of the LV and the RV, even in theabsence of intrinsic AV conduction of the heart.

In some proposed adaptive CRT pacing techniques, a pacing configuration(e.g., fusion pacing or biventricular pacing) and timing of the pacingpulses (e.g., a fusion pacing interval, such as a A_(P/S)−V2_(P) delay,or biventricular pacing interval, such as a V1_(P)−V2_(P) delay) areperiodically adjusted based on periodic intrinsic conductionmeasurements in an attempt to achieve more efficient physiologic pacingand optimal hemodynamics. For example, some proposed cardiac rhythmmanagement medical devices are configured to deliver adaptive CRT bydelivering pacing to a heart of a patient in accordance with a fusionpacing configuration and, if loss of intrinsic AV conduction is detected(e.g., AV block), switching to a biventricular pacing configuration.Thus, a medical device configured for adaptive CRT may be configured toswitch from a fusion pacing configuration to a biventricular pacingconfiguration in response to determining a heart of a patient is nolonger intrinsically conducting. Biventricular pacing may consume moreenergy (compared to fusion pacing) due to the delivery of pacing to boththe LV and the RV, and, accordingly, delivering fusion pacing until lossof intrinsic conduction may be a more efficient use of the power storedby a power source of a medical device compared to continuouslydelivering biventricular pacing.

In some existing proposed techniques for delivering adaptive CRT, amedical device switches from a fusion pacing configuration to abiventricular pacing configuration if the loss of intrinsic AVconduction is detected based on a measurement of intrinsic conductiontime, which may be performed as part of the fusion-pacing intervaldetermination. For example, loss of intrinsic AV conduction may bedetected if a measured A−V1 conduction time (A_(P/S)−V1_(S)) is greaterthan (or greater than or equal to in some examples) a predeterminedthreshold value. In some examples, the predetermined threshold value isselected based on previous intrinsic conduction time intervals (e.g.,may be a percentage of a mean or median of a certain number of priorintrinsic conduction time measurements). In other examples, thepredetermined threshold value may be selected by a clinician to be, forexample, a value that indicates the depolarization time of V1 thatresults maintenance of cardiac output at a desirable level.

Depending on the PEI used to time the fusion pacing pulses, in order tomeasure intrinsic conduction time, the pacing delivered by the medicaldevice to the heart may be suspended to allow the heart of the patientto conduct in the absence of cardiac rhythm management therapy and toavoid interference between the delivery of pacing pulses and sensing ofventricular activation. In some examples, if pacing is delivered to anatrium of the heart, such pacing may be maintained, while pacing to theventricles may be suspended. The measurement of intrinsic conductiontime may be determined, e.g., as the time between an atrial pace orsense event (A_(P/S)) and a V1 sensing event (V1_(S)). Thedeterminations of the intrinsic conduction time measurements may takeplace, for example, once a minute, once an hour, or once every 24 hours,although other frequencies may also be used.

The determinations of intrinsic conduction time may involve thesuspension of some or all pacing therapy to the heart of the patient forat least one cardiac cycle, which may reduce the amount ofsynchronization of the ventricles of the heart during at least that onecardiac cycle. In addition, between the measurements of intrinsicconduction times, the intrinsic conduction times are assumed to stayconstant. Further, if intrinsic conduction was detected during onemeasurement of intrinsic conduction time, the medical device isconfigured to assume that the heart is intrinsically conducting untilthe next intrinsic conduction time measurement, at which point themedical device may reassess intrinsic conduction. Accordingly, thepacing configuration of the medical device may only be switched asfrequently as the intrinsic conduction time determinations are made.

If an abrupt complete AV block develops between two consecutiveintrinsic conduction time measurements and is coupled with a loss of V2pacing capture, a switch from the fusion pacing configuration to thebiventricular pacing configuration may help minimize the possibility ofa drop in the heart rate and cardiac output of the heart of the patient.However, in these existing proposed techniques for adaptive CRT, theswitch to biventricular pacing configuration may not occur until thenext intrinsic AV conduction time determination, which may be some timeafter the loss of intrinsic AV conduction and/or the loss of V2 captureby the medical device.

In comparison to these existing proposed techniques for deliveringadaptive CRT, the devices, systems, and techniques described herein mayprovide a more responsive switch from a fusion pacing configuration to abiventricular pacing configuration when a loss of intrinsic AVconduction is present. As described herein, the devices, systems, andtechniques described herein for providing adaptive CRT are directed toswitching pacing configurations based on a surrogate indication of thepresence or absence of intrinsic AV conduction, which is determinedwithout the need to suspend delivery of electrical stimulation to theheart of the patient. The surrogate indication of intrinsic conductionmay be used to verify V1 ventricular activation after, e.g., every pacedbeat or at least more frequently than the AV conduction timemeasurements. It may be desirable to confirm that the heart of thepatient is intrinsically depolarizing in order to confirm that fusionpacing is still the proper pacing configuration for the patient. In someexamples, the time delay between the atrial pace or sense event and theV2 sense event detected within the predetermined window of timefollowing the pacing stimulus (V2_(P)), referred to herein as anA_(P/S)−V1_(S) interval, may be used to as an AV conduction time that isused to adjust a pacing interval.

In some examples, while a medical device is delivering fusion pacing toa heart of a patient, the medical device may determine whether the heartis intrinsically conducting (e.g., from the right atrium to the firstcontracting ventricle V1) based on the surrogate indication of intrinsicAV conduction. In this way, the surrogate indication of intrinsic AVconduction may indicate whether a fusion pacing configuration is stillappropriate for a patient. In some examples, the surrogate indication isdetermined to be a detection of ventricular activation (V_(1S)) of thefirst contracting ventricle (V1) within a predetermined window of timeimmediately following the delivery of a pacing stimulus to the latercontracting ventricle (V_(2P)). The pacing stimulus (V_(2P)) may be, forexample, a fusion pacing pulse that is delivered as part of fusionpacing therapy.

It is believed that the pacing stimulus (V2_(P)) may dissipate prior topropagating to the first contracting ventricle (V1), such thatdepolarization of the first contracting ventricle (V1_(S)) following thedelivery of the pacing stimulus to the later contracting ventricle(V2_(P)) is attributable to intrinsic atrioventricular conduction, e.g.,from the right atrium, left atrium, or both, via the atrioventricular(AV) node. As discussed in further detail below, e.g., with reference toFIGS. 7-11, the medical device may confirm that the sense event of thefirst contracting ventricle (V1_(S)) is not attributable to the pacingpulse that is delivered to the later contracting ventricle (V2_(P)),which may indicate the sense event of the other ventricle (V1_(S)) isattributable to intrinsic conduction. In some cases, a failure to detectthe surrogate indication of intrinsic AV conduction for a certain numberof beats may indicate a loss of intrinsic AV conduction.

If the medical device detects a loss of intrinsic AV conduction (e.g.,because of a failure to detect the V1 ventricular activation afterdelivery of a pacing pulse to the V2 or a failure to detect the V1ventricular activation for a certain number of beats), the medicaldevice is configured to take one or more various responsive actions. Insome examples, in response to detecting loss of intrinsic AV conductionbased on the failure to detect the surrogate indication of intrinsic AVconduction, the medical device is configured to determine to suspendelectrical stimulation to the heart and perform an intrinsic AVconduction time measurement in order to determine (e.g., verify) whetherintrinsic AV conduction is present. The intrinsic AV conduction timemeasurement may include, for example, suspending or delaying allelectrical stimulation to the ventricles of the heart of the patient anddetermining the time between an atrial pace or sense event (A_(P/S)) andactivation of the first contracting ventricle (V1_(S)). In response todetermining the intrinsic AV conduction time measurement indicatesintrinsic AV conduction is present (e.g., that the V1 is activating inthe absence of electrical stimulation to heart the heart), the medicaldevice may maintain the fusion pacing configuration. On the other hand,in response to determining the intrinsic AV conduction measurementindicates intrinsic AV conduction is not present, the medical device mayterminate the fusion pacing and switch to a biventricular pacingconfiguration.

In other examples, the medical device is configured to automaticallyterminate the fusion pacing and switch to a biventricular pacingconfiguration without performing an intrinsic AV conduction timedetermination if the medical device detects a loss of intrinsic AVconduction based on an absence of the surrogate indication of intrinsicAV conduction after the delivery of a fusion pacing pulse to the laterdepolarizing ventricle V2.

The techniques described herein for controlling adaptive CRT may be usedwith dual chamber or triple chamber medical devices, where sensing andpacing vectors are the same or different.

FIG. 1 is a conceptual diagram illustrating an example therapy system 10that is configured to provide electrical stimulation therapy to heart 12of patient 14. Therapy system 10 includes IMD 16, which is coupled toleads 18, 20, and 22, and programmer 24. IMD 16 may be, for example, animplantable pacemaker that provides electrical signals to heart 12 andsenses electrical activity of heart 12 via electrodes coupled to one ormore of leads 18, 20, and 22. In some examples, IMD 16 may includecardioversion and/or defibrillation capabilities.

Leads 18, 20, 22 extend into the heart 12 of patient 14 to senseelectrical activity of heart 12 and deliver electrical stimulation toheart 12. In the example shown in FIG. 1, right ventricular (RV) lead 18extends through one or more veins (not shown), the superior vena cava(not shown), and right atrium (RA) 26, and into RV 28. Left ventricular(LV) coronary sinus lead 20 extends through one or more veins, the venacava, right atrium 26, and into the coronary sinus 30 to a regionadjacent to the free wall of LV 32 of heart 12. Right atrial (RA) lead22 extends through one or more veins and the vena cava, and into the RA26 of heart 12.

IMD 16 may sense electrical signals attendant to the depolarization andrepolarization of heart 12 via electrodes (not shown in FIG. 1) coupledto at least one of the leads 18, 20, 22. In some examples, IMD 16 mayalso sense electrical signals attendant to the depolarization andrepolarization of heart 12 via extravascular electrodes (e.g., outsidethe vasculature of patient 14), such as epicardial electrodes, externalsurface electrodes, subcutaneous electrodes, and the like. In someexamples, as described in further detail below, IMD 16 provides pacingpulses to heart 12 based on the electrical signals sensed within heart12. For example, IMD 16 may provide pacing pulses to LV 32 based on theelectrical signals sensed within RV 28. The configurations of electrodesused by IMD 16 for sensing and pacing may be unipolar or bipolar.

IMD 16 is configured to provide adaptive CRT to heart 12. In someexamples, as part of the adaptive CRT, IMD 16 is configured to deliveryat least one of fusion pacing to heart 12 and biventricular pacingtherapy to heart 12. In some examples of fusion pacing, IMD 16 maydeliver a pacing stimulus (e.g., a pacing pulse) to LV 32 via electrodesof lead 20, where the pacing stimulus is timed such that an evokeddepolarization of LV 32 is effected in fusion with the intrinsicdepolarization of RV 28, resulting in a ventricular resynchronization.In this way, the pacing pulse delivered to LV 32 (LV_(P)) may pre-excitea conduction delayed LV 32 and help fuse the activation of LV 32 withthe activation of RV 28 from intrinsic conduction. The fusion of thedepolarization of LV 32 and RV 28 may result in synchronous activationand contraction of LV 32 with RV 28. In some examples, when IMD 16 is ina biventricular pacing configuration, IMD 16 may deliver a pacingstimulus (e.g., a pacing pulse) to RV 28 via electrodes of lead 18 and apacing stimulus to LV 32 via electrodes of lead 20 in a manner thatsynchronizes activation and contraction of RV 28 and LV 28.

As discussed in further detail below, in some examples, IMD 16 may beconfigured to adapt the pacing configuration to the cardiac status ofheart 12 by delivering electrical stimulation therapy to heart 12according to the fusion pacing configuration and, when an absence ofintrinsic AV conduction (e.g., from RA 26 to RV 28) is detected, adjusta pacing parameter, e.g., by switching to a biventricular pacingconfiguration or another multisite pacing, or increasing the pacingoutput (e.g., the frequency of pacing pulses or the intensity of thepacing pulses, such as the current or voltage amplitude). While IMD 16may periodically measure intrinsic AV conduction times to confirm thepresence of intrinsic AV conduction, IMD 16 may more frequentlydetermine that intrinsic AV conduction is absent based on surrogateindication of the intrinsic AV conduction time of heart 12. In this way,the surrogate indication of intrinsic AV conduction may indicate whethera fusion pacing configuration is still appropriate for a patient.

The surrogate indication is not an actual measurement of intrinsic AVconduction time, but is an indication that RV 28 has intrinsicallyactivated. The surrogate indication may be an occurrence of ventricularsense event of RV 28 (RV_(S)), which indicates activation of RV 28,within a predetermined window of time (e.g., about 30 milliseconds toabout 100 milliseconds) after delivery of a pacing pulse to LV 32(LV_(P)). The ventricular sense event of RV 28 can be, for example, anR-wave of an electrical cardiac signal sensed via electrodes of lead 18.If the blanking interval with which IMD 16 senses electrical cardiacactivity of heart 12 following the LV pace event (LV_(P)) is greaterthan the predetermined window of time, the blanking window may beshortened in some examples in order to detect the surrogate indicationof the intrinsic AV conduction. In some examples, IMD 16 determines atime delay between the time IMD 16 delivers a pacing pulse to LV 32(LV_(P)) and a subsequent sense event of RV 28 (RV_(S)).

In some cases, the electrical activation of LV 32 caused by delivery ofthe pacing pulse to LV 32 (LV_(P)) may propagate to RV 28, and maypresent itself in an electrical cardiac signal (e.g., anelectrocardiogram (ECG) or cardiac electrogram (EGM)) as an RV senseevent RV_(S), and, therefore, the RV sense event RV_(S) may not berepresentative of intrinsic AV conduction. In order to confirm that theRV sense event RV_(S) is representative of intrinsic AV conduction, IMD16 may confirm that the sense event of RV 28 (RV_(S)) is notattributable to the pacing pulse is delivered to LV 32 (LV_(P)). Exampletechniques for confirming that the sense event of RV 28 (RV_(S)) is notattributable to the pacing pulse is delivered to LV 32 (LV_(P)) aredescribed in further detail below with reference to FIGS. 7-11.

The adaptive CRT provided by IMD 16 may be useful for maintaining thecardiac rhythm in patient 14 with a conduction dysfunction, which mayresult when the natural electrical activation system of heart 12 isdisrupted. The natural electrical activation system of a human heart 12involves several sequential conduction pathways starting with thesino-atrial (SA) node, and continuing through the atrial conductionpathways of Bachmann's bundle and internodal tracts at the atrial level,followed by the atrio-ventricular (AV) node, Common Bundle of His, rightand left bundle branches, and a final distribution to the distalmyocardial terminals via the Purkinje fiber network.

In a normal electrical activation sequence, the cardiac cycle commenceswith the generation of a depolarization wave at the SA Node in the wallof RA 26. The depolarization wave is transmitted through the atrialconduction pathways of Bachmann's Bundle and the Internodal Tracts atthe atrial level into the LA 33 septum. When the atrial depolarizationwave has reached the AV node, the atrial septum, and the furthest wallsof the right and left atria 26, 33, respectively, the atria 26, 33 maycontract as a result of the electrical activation. The aggregate rightatrial and left atrial depolarization wave appears as the P-wave of thePQRST complex of an electrical cardiac signal, such as an EGM or ECG.When the amplitude of the atrial depolarization wave passing between apair of unipolar or bipolar pace/sense electrodes located on or adjacentRA 26 and/or LA 33 exceeds a threshold, it is detected as a sensedP-wave. The sensed P-wave may also be referred to as an atrial sensingevent, or an RA sensing event (RA_(S)). Similarly, a P-wave sensed inthe LA 33 may be referred to as an atrial sensing event or an LA sensingevent (LA_(S)).

During or after the atrial contractions, the AV node distributes thedepolarization wave inferiorly down the Bundle of His in theintraventricular septum. The depolarization wave may travel to theapical region of heart 12 and then superiorly though the Purkinje Fibernetwork. The aggregate right ventricular and left ventriculardepolarization wave and the subsequent T-wave accompanyingre-polarization of the depolarized myocardium may appear as the QRSTportion of the PQRST cardiac cycle complex. When the amplitude of theQRS ventricular depolarization wave passing between a bipolar orunipolar pace/sense electrode pair located on or adjacent RV 28 and/orLV 32 exceeds a threshold, it is detected as a sensed R-wave. The sensedR-wave may also be referred to as a ventricular sensing event, an RVsensing event (RV_(S)), or an LV sensing event (LV_(S)) depending uponthe ventricle in which the electrodes of one or more of leads 18, 20, 22are configured to sense in a particular case.

Some patients, such as patients with congestive heart failure orcardiomyopathies, may have left ventricular dysfunction, whereby thenormal electrical activation sequence through heart 12 is compromisedwithin LV 32. In a patient with left ventricular dysfunction, the normalelectrical activation sequence through the heart of the patient becomesdisrupted. For example, patients may experience an intra-atrialconduction defect, such as intra-atrial block. Intra-atrial block is acondition in which the atrial activation is delayed because ofconduction delays between RA 26 to LA 33.

As another example, a patient with left ventricular dysfunction mayexperience an interventricular conduction defect, such as left bundlebranch block (LBBB) and/or right bundle branch block (RBBB). In LBBB andRBBB, the activation signals are not conducted in a normal fashion alongthe right or left bundle branches respectively. Thus, in patients withbundle branch block, the activation of either RV 28 or LV 32 is delayedwith respect to the other ventricle, causing asynchrony between thedepolarization of the right and left ventricles. Ventricular asynchronymay be identified by a widened QRS complex due to the increased time forthe activation to traverse the ventricular conduction paths. Theasynchrony may result from conduction defects along the Bundle of His,the Right and Left Bundle Branches or at the more distal PurkinjeTerminals. Typical intra-ventricular peak-to-peak asynchrony can rangefrom about 80 ms to about 200 ms or longer. However, in patients who areexperiencing RBBB and LBBB, the QRS complex may be widened far beyondthe normal range to a wider range, e.g., about 120 ms to about 250 ms orgreater.

CRT delivered by IMD 16 may help alleviate heart failure conditions byrestoring synchronous depolarization and contraction of one or morechambers of heart 12. In some cases, the fusion pacing of heart 12described herein enhances stroke volume of a patient by improving thesynchrony with which RV 28 and LV 32 depolarize and contract.

The duration of a cardiac cycle of heart 12, which includes thedepolarization-repolarization sequence, may change depending on variousphysiological factors of patient 14, such as a heart rate. As heart rateof patient 14 changes, the timing of the delivery of a pacing pulse toLV 32 (LV_(P)) during fusion pacing therapy or the timing of thedelivery of pacing pulses to RV 28 (RV_(P)) and LV 32 (LV_(P)) duringbiventricular pacing therapy may change. Accordingly, when IMD 16 isdelivering fusion pacing to heart 12, it may be useful for IMD 16 toperiodically adjust a fusion pacing interval in order to maintain thedelivery of the LV 32 pacing pulse (LV_(P)) at a time that results in afusion of the depolarization of LV 32 and RV 28. In addition, when IMD16 is delivering biventricular pacing therapy to heart 12, it may beuseful for IMD 16 to periodically evaluate a biventricular pacinginterval in order to maintain the delivery of the LV 32 pacing pulse(LV_(P)) at a time relative to the RV 28 pacing pulse (RV_(P)) thatresults in a synchrony of contraction of LV 32 and RV 28. As discussedin further detail below, in some examples, IMD 16 adjusts the fusionpacing interval based on the A_(P/S)−RV_(S) interval of a cardiac cycle,where the RV sense event (RV_(S)) is sensed within the predeterminedwindow of time immediately following the LV 32 pacing pulse (LV_(P)).

In some examples, IMD 16 delivers pacing pulses to LV 32 until a loss ofcapture occurs in LV 32. Depolarization of the myocardial cells inresponse to a pacing pulse may be referred to as “capture.” Detection ofloss of capture in LV 32 may indicate that the LV pacing pulse (LV_(P))is being delivered too late (e.g., during the refractory period of LV32) or that the LV 32 pacing electrodes have become dislodged or lead 20has a lead-related condition (e.g., comprised insulation or a fracture).If the loss of capture is detected, e.g., by a failure to detect an LVsensing event (LV_(S)) within a predetermined amount of time followingthe delivery of a LV pacing pulse, IMD 16 may discontinue the fusionpacing therapy. In some cases, IMD 16 switches to a different pacingmode (e.g., an AAI, ADI, AAI/R, ADI/R, double chamber DDD or DDD/R, andthe like) after discontinuing the fusion pacing therapy.

In some examples, IMD 16 also provides defibrillation therapy and/orcardioversion therapy via electrodes located on at least one of theleads 18, 20, 22. IMD 16 may detect arrhythmia of heart 12, such asfibrillation of ventricles 28 and 32, and deliver defibrillation therapyto heart 12 in the form of electrical pulses. In some examples, IMD 16is programmed to deliver a progression of therapies, e.g., pulses withincreasing energy levels, until a fibrillation of heart 12 is stopped.IMD 16 detects fibrillation employing one or more fibrillation detectiontechniques known in the art.

In some examples, programmer 24 may be a handheld computing device or acomputer workstation. Programmer 24 may include a user interface thatreceives input from a user. The user interface may include, for example,a keypad and a display, which may for example, be a cathode ray tube(CRT) display, a liquid crystal display (LCD) or light emitting diode(LED) display. The keypad may take the form of an alphanumeric keypad ora reduced set of keys associated with particular functions. Programmer24 can additionally or alternatively include a peripheral pointingdevice, such as a mouse, via which a user may interact with the userinterface. In some embodiments, a display of programmer 24 may include atouch screen display, and a user may interact with programmer 24 via thedisplay.

A user, such as a physician, technician, or other clinician, mayinteract with programmer 24 to communicate with IMD 16. For example, theuser may interact with programmer 24 to retrieve physiological ordiagnostic information from IMD 16. A user may also interact withprogrammer 24 to program IMD 16, e.g., select values for operationalparameters of the IMD.

For example, the user may use programmer 24 to retrieve information fromIMD 16 regarding the rhythm of heart 12, trends therein over time, ortachyarrhythmia episodes. As another example, the user may useprogrammer 24 to retrieve information from IMD 16 regarding other sensedphysiological parameters of patient 14, such as sensed electricalactivity, intracardiac or intravascular pressure, activity, posture,respiration, or thoracic impedance. As another example, the user may useprogrammer 24 to retrieve information from IMD 16 regarding theperformance or integrity of IMD 16 or other components of system 10,such as leads 18, 20, and 22, or a power source of IMD 16.

The user may use programmer 24 to program a therapy progression, selectelectrodes used to deliver defibrillation pulses, select waveforms forthe defibrillation pulse, or select or configure a fibrillationdetection algorithm for IMD 16. The user may also use programmer 24 toprogram aspects of other therapies provided by IMD 14, such ascardioversion or pacing therapies. In some examples, the user mayactivate certain features of IMD 16 by entering a single command viaprogrammer 24, such as depression of a single key or combination of keysof a keypad or a single point-and-select action with a pointing device.

IMD 16 and programmer 24 may communicate via wireless communicationusing any techniques known in the art. Examples of communicationtechniques may include, for example, low frequency or radiofrequency(RF) telemetry, but other techniques are also contemplated. In someexamples, programmer 24 may include a programming head that may beplaced proximate to the patient's body near the IMD 16 implant site inorder to improve the quality or security of communication between IMD 16and programmer 24.

FIG. 2 is a conceptual diagram illustrating IMD 16 and leads 18, 20, 22of therapy system 10 in greater detail. Leads 18, 20, 22 may beelectrically coupled to a stimulation generator, a sensing module, orother modules IMD 16 via connector block 34. In some examples, proximalends of leads 18, 20, 22 include electrical contacts that electricallycouple to respective electrical contacts within connector block 34. Inaddition, in some examples, leads 18, 20, 22 are mechanically coupled toconnector block 34 with the aid of set screws, connection pins oranother suitable mechanical coupling mechanism.

Each of the leads 18, 20, 22 includes an elongated insulative lead body,which may carry a number of conductors separated from one another bytubular insulative sheaths. In the illustrated example, bipolarelectrodes 40 and 42 are located proximate to a distal end of lead 18.In addition, bipolar electrodes 44 and 46 are located proximate to adistal end of lead 20 and bipolar electrodes 48 and 50 are locatedproximate to a distal end of lead 22. Electrodes 40, 44, and 48 may takethe form of ring electrodes, and electrodes 42, 46 and 50 may take theform of extendable helix tip electrodes mounted retractably withininsulative electrode heads 52, 54 and 56, respectively. Each of theelectrodes 40, 42, 44, 46, 48 and 50 may be electrically coupled to arespective one of the conductors within the lead body of its associatedlead 18, 20, 22, and thereby coupled to respective ones of theelectrical contacts on the proximal end of leads 18, 20 and 22.

Electrodes 40, 42, 44, 46, 48 and 50 may sense electrical signalsattendant to the depolarization and repolarization of heart 12. Theelectrical signals are conducted to IMD 16 via the respective leads 18,20, 22. In some examples, IMD 16 also delivers pacing pulses to LV 32via electrodes 44, 46 to cause depolarization of cardiac tissue of heart12. In some examples, as illustrated in FIG. 2, IMD 16 includes one ormore housing electrodes, such as housing electrode 58, which may beformed integrally with an outer surface of hermetically-sealed housing60 of IMD 16 or otherwise coupled to housing 60. In some examples,housing electrode 58 is defined by an uninsulated portion of an outwardfacing portion of housing 60 of IMD 16. Other division between insulatedand uninsulated portions of housing 60 may be employed to define two ormore housing electrodes. In some examples, housing electrode 58comprises substantially all of housing 60. Any of the electrodes 40, 42,44, 46, 48, and 50 may be used for unipolar sensing or stimulationdelivery in combination with housing electrode 58. As described infurther detail with reference to FIG. 3, housing 60 may enclose astimulation generator that generates cardiac pacing pulses anddefibrillation or cardioversion shocks, as well as a sensing module formonitoring the patient's heart rhythm.

In some examples, leads 18, 20, 22 may also include elongated electrodes62, 64, 66, respectively, which may take the form of a coil. IMD 16 maydeliver defibrillation pulses to heart 12 via any combination ofelongated electrodes 62, 64, 66, and housing electrode 58. Electrodes58, 62, 64, 66 may also be used to deliver cardioversion pulses to heart12. Electrodes 62, 64, 66 may be fabricated from any suitableelectrically conductive material, such as, but not limited to, platinum,platinum alloy or other materials known to be usable in implantabledefibrillation electrodes.

The configuration of therapy system 10 illustrated in FIGS. 1 and 2 ismerely one example. In other examples, a therapy system may includeextravascular electrodes, such as subcutaneous electrodes, epicardialelectrodes, and/or patch electrodes, instead of or in addition to theelectrodes of transvenous leads 18, 20, 22 illustrated in FIG. 1.Further, IMD 16 need not be implanted within patient 14. In examples inwhich IMD 16 is not implanted in patient 14, IMD 16 may deliverdefibrillation pulses, pacing pulses, and other therapies to heart 12via percutaneous leads that extend through the skin of patient 14 to avariety of positions within or outside of heart 12.

In other examples of therapy systems that provide electrical stimulationtherapy to heart 12, a therapy system may include any suitable number ofleads coupled to IMD 16, and each of the leads may extend to anylocation within or proximate to heart 12. For example, a therapy systemmay include a single chamber or dual chamber device rather than athree-chamber device as shown in FIG. 1. In a single chamberconfiguration, IMD 16 is electrically connected to a single lead 20 thatincludes stimulation and sense electrodes within LV 32. In one exampleof a dual chamber configuration, IMD 16 is electrically connected to asingle lead that includes stimulation and sense electrodes within LV 32as well as sense and/or stimulation electrodes within RA 26, as shown inFIG. 3. In another example of a dual chamber configuration, IMD 16 isconnected to two leads that extend into a respective one of RA 28 and LV32. Other lead configurations are contemplated.

FIG. 3 is a functional block diagram of one example configuration of IMD16, which includes processor 80, memory 82, stimulation generator 84,sensing module 86, telemetry module 88, and power source 90. Memory 82includes computer-readable instructions that, when executed by processor80, cause IMD 16 and processor 80 to perform various functionsattributed to IMD 16 and processor 80 herein. Memory 82 may include anyvolatile, non-volatile, magnetic, optical, or electrical media, such asa random access memory (RAM), read-only memory (ROM), non-volatile RAM(NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory,or any other digital media. In addition to sensed physiologicalparameters of patient 14 (e.g., EGM or ECG signals), one or more timeintervals for timing fusion pacing therapy and biventricular pacingtherapy to heart 12 (e.g., PEI values, adjusted PEI values,biventricular pacing intervals, or any combination thereof) may bestored by memory 82.

Processor 80 may include any one or more of a microprocessor, acontroller, a digital signal processor (DSP), an application specificintegrated circuit (ASIC), a field-programmable gate array (FPGA), orequivalent discrete or integrated logic circuitry. In some examples,processor 80 may include multiple components, such as any combination ofone or more microprocessors, one or more controllers, one or more DSPs,one or more ASICs, or one or more FPGAs, as well as other discrete orintegrated logic circuitry. The functions attributed to processor 80herein may be embodied as software, firmware, hardware or anycombination thereof. Processor 80 is configured to control stimulationgenerator 84 to deliver stimulation therapy to heart 12 according to aselected one or more of therapy programs, which may be stored in memory82. For example, processor 80 may be configured to stimulation generator84 to deliver electrical pulses with the amplitudes, pulse widths,frequency, or electrode polarities specified by the selected one or moretherapy programs.

Stimulation generator 84 is electrically coupled to electrodes 40, 42,44, 46, 48, 50, 58, 62, 64, and 66, e.g., via conductors of therespective lead 18, 20, 22, or, in the case of housing electrode 58, viaan electrical conductor disposed within housing 60 of IMD 16.Stimulation generator 84 is configured to generate and deliverelectrical stimulation therapy. For example, stimulation generator 84may deliver a pacing stimulus to LV 32 (FIG. 2) of heart 12, e.g., inaccordance with the fusion pacing techniques described herein, via atleast two electrodes 44, 46 (FIG. 2). As another example, stimulationgenerator 84 may deliver a pacing stimulus to RV 28 via at least twoelectrodes 40, 42 (FIG. 2) and a pacing stimulus to LV 32 via at leasttwo electrodes 44, 46 (FIG. 2), e.g., in accordance with thebiventricular pacing techniques described herein.

In some examples, stimulation generator 84 is configured to delivercardioversion or defibrillation shocks to heart 12. The pacing stimuli,cardioversion shocks, and defibrillation shocks may be in the form ofstimulation pulses. In other examples, stimulation generator may deliverone or more of these types of stimulation in the form of other signals,such as sine waves, square waves, or other substantially continuous timesignals.

Stimulation generator 84 may include a switch module, and processor 80may use the switch module to select, e.g., via a data/address bus, whichof the available electrodes are used to deliver defibrillation pulses orpacing pulses. The switch module may include a switch array, switchmatrix, multiplexer, or any other type of switching device suitable toselectively couple stimulation energy to selected electrodes. In otherexamples, processor 80 may select a subset of electrodes 40, 42, 44, 46,48, 50, 58, 62, 64, and 66 with which stimulation is delivered to heart12 without a switch module.

Sensing module 86 is configured to monitor signals from at least one ofelectrodes 40, 42, 44, 46, 48, 50, 58, 62, 64 or 66 in order to monitorelectrical activity of heart 12, e.g., via EGM signals. For example,sensing module 86 may sense atrial events (e.g., a P-wave) withelectrodes 48, 50, 66 within RA 26 or sense an LV 32 event (e.g., anR-wave) with electrodes 44, 46, 64 within LV 32. In some examples,sensing module 86 includes a switch module to select which of theavailable electrodes are used to sense the heart activity. For example,processor 80 may select the electrodes that function as sense electrodesvia the switch module within sensing module 86, e.g., by providingsignals via a data/address bus. In some examples, sensing module 86includes one or more sensing channels, each of which may comprises anamplifier. In response to the signals from processor 80, the switchmodule of within sensing module 86 may couple the outputs from theselected electrodes to one of the sensing channels.

In some examples, one channel of sensing module 86 may include an R-waveamplifier that receives signals from electrodes 40 and 42, which areused for pacing and sensing in RV 28 of heart 12. Another channel mayinclude another R-wave amplifier that receives signals from electrodes44 and 46, which are used for pacing and sensing proximate to LV 32 ofheart 12. In some examples, the R-wave amplifiers may take the form ofan automatic gain controlled amplifier that provides an adjustablesensing threshold as a function of the measured R-wave amplitude of theheart rhythm.

In addition, in some examples, one channel of sensing module 86 mayinclude a P-wave amplifier that receives signals from electrodes 48 and50, which are used for pacing and sensing in RA 26 of heart 12. In someexamples, the P-wave amplifier may take the form of an automatic gaincontrolled amplifier that provides an adjustable sensing threshold as afunction of the measured P-wave amplitude of the heart rhythm. Examplesof R-wave and P-wave amplifiers are described in U.S. Pat. No. 5,117,824to Keimel et al., which issued on Jun. 2, 1992 and is entitled,“APPARATUS FOR MONITORING ELECTRICAL PHYSIOLOGIC SIGNALS,” and isincorporated herein by reference in its entirety. Other amplifiers mayalso be used. Furthermore, in some examples, one or more of the sensingchannels of sensing module 86 may be selectively coupled to housingelectrode 58, or elongated electrodes 62, 64, or 66, with or instead ofone or more of electrodes 40, 42, 44, 46, 48 or 50, e.g., for unipolarsensing of R-waves or P-waves in any of chambers 26, 28, or 32 of heart12.

In some examples, sensing module 86 includes a channel that comprises anamplifier with a relatively wider pass band than the R-wave or P-waveamplifiers. Signals from the selected sensing electrodes that areselected for coupling to this wide-band amplifier may be provided to amultiplexer, and thereafter converted to multi-bit digital signals by ananalog-to-digital converter for storage in memory 82 as an EGM. In someexamples, the storage of such EGMs in memory 82 may be under the controlof a direct memory access circuit. Processor 80 may employ digitalsignal analysis techniques to characterize the digitized signals storedin memory 82 to detect and classify the patient's heart rhythm from theelectrical signals. Processor 80 may detect and classify the heartrhythm of patient 14 by employing any of the numerous signal processingmethodologies known in the art.

Signals generated by sensing module 86 may include, for example: anRA-event signal, which indicates a detection of a P-wave via electrodesimplanted within RA 26 (FIG. 1); an LA-event signal, which indicates adetection of a P-wave via electrodes implanted within LA 33 (FIG. 1); anRV-event signal, which indicates a detection of an R-wave via electrodesimplanted within RV 28; or an LV-event signal, which indicates adetection of an R-wave via electrodes implanted within LV 32. In theexample of therapy system 10 shown in FIGS. 1 and 2, IMD 16 is notconnected to electrodes that are implanted within LA 33. However, inother example therapy systems, IMD 16 may be connected to electrodesthat are implanted within LA 33 in order to sense electrical activity ofLA 33.

Processor 80 may define variable intervals for timing the delivering ofpacing pulses to heart 12 based on one or more signals sensed by sensingmodule 86. For example, processor 80 may define variable intervals fortiming the delivery of LV fusion pacing pulses (LV_(P)) based on signalsfrom sensing module 86. These intervals may include, for example, afusion pacing interval (e.g., A_(P/S)−LV_(P) delay or LV_(P)−LV_(S)delay) and intervals used to determine the fusion pacing intervals orother pacing intervals (e.g., A_(P/S)−LV_(S), RV_(S)−LV_(S), orLV_(S)−LV_(S)). Example techniques for determining the fusion pacingintervals (A_(P/S)−LV_(P)) is described in U.S. Pat. No. 7,181,284 toBurnes et al. and U.S. Patent Application Publication No. 2010/0198291by Sambelashvili et al. As another example, processor 80 may definevariable intervals for timing the delivery of RV 28 and LV 32 pacingpulses during the delivery of biventricular pacing therapy based on oneor more signals sensed by sensing module 86. These intervals mayinclude, for example, an interval for pacing RV 28 relative to an atrialpace or sense event (e.g., the A_(P/S)−RV_(P) delay) or an interval forpacing LV 32 relative to the RV 28 pace event (e.g., the RV_(P)−LV_(P)delay).

Processor 80 includes pacer timing and control module 92, which may beembodied as hardware, firmware, software, or any combination thereof.Pacer timing and control module 92 may comprise a dedicated hardwarecircuit, such as an ASIC, separate from other processor 80 components,such as a microprocessor, or a software module executed by a componentof processor 80 (e.g., a microprocessor or ASIC). Pacer timing andcontrol module 92 may help define the pacing interval (e.g.,A_(P/S)−LV_(P) delay, LV_(S)−LV_(P) delay, A_(P/S)−RV_(P) delay, or theRV_(P)−LV_(P) delay) for controlling the delivery of a pacing pulse toLV 32. For example, pacing timing and control module 92 may includeprogrammable counters or timers for determining the A_(P/S)−LV_(P)delay, the LV_(S)−LV_(P) delay, and/or any other relevant timeintervals. In addition, pacing timing and control module 92 may includetimers for timing the delivery of pacing pulses and other functions thatare based on the pacing interval. For example, in examples of fusionpacing in which IMD 16 delivers the LV pacing pulse (LV_(P)) apredetermined period of time following an atrial pace or sense event(A_(P/S)), pacing timing and control module 92 may include a timer thatis loaded with the appropriate A_(P/S)−LV_(P) delay. The timer of pacingtiming and control module 92 may be configured to begin upon thedetection of a preceding atrial pace or sense event (A_(P/S)). Uponexpiration of the particular timer, processor 80 may control stimulationgenerator 84 to deliver pacing stimulus LV_(P) to LV 32. For example,pacing timing and control module 92 may generate a trigger signal thattriggers the output of a pacing pulse by stimulation generator 84.

As another example, in some examples of fusion pacing in which IMD 16delivers the LV pacing pulse (LV_(P)) a predetermined period of timefollowing a LV 32 sensing event (LV_(S)), pacing timing and controlmodule 92 may include a timer that is loaded with the appropriateLV_(S)−LV_(P) delay. The timer of pacing timing and control module 92may be configured to begin upon detection of a preceding LV sensingevent (LV_(S)). Upon expiration of the particular timer, processor 80may control stimulation generator 84 to deliver pacing stimulus LV_(P)to LV 32 (FIG. 1).

In some examples of biventricular pacing in which IMD 16 delivers apacing pulse to RV 28 a predetermined period of time following an atrialpace or sense event (A_(P/S)), pacing timing and control module 92 mayinclude a timer that is loaded with the appropriate A_(P/S)−RV_(P)delay. The timer of pacing timing and control module 92 may beconfigured to begin upon detection of a preceding atrial pace or senseevent (LV_(S)). Upon expiration of the particular timer, processor 80may control stimulation generator 84 to deliver pacing stimulus RV_(P)to RV 28 (FIG. 1). In addition, in some examples of biventricular pacingin which IMD 16 delivers a pacing pulse to LV 28 a predetermined periodof time following right ventricular pacing event (RV_(P)), pacing timingand control module 92 may include a timer that is loaded with theappropriate RV_(P)−LV_(P) delay. The timer of pacing timing and controlmodule 92 may be configured to begin upon detection of a RV pacing event(RV_(P)). Upon expiration of the particular timer, processor 80 maycontrol stimulation generator 84 to deliver pacing stimulus LV_(P) to LV32 (FIG. 1).

In examples in which IMD 16 is configured to deliver other types ofcardiac rhythm therapy in addition to fusion pacing and biventricularpacing, pacer timing and control module 92 may also include programmablecounters which control the basic time intervals associated with DDD,VVI, DVI, VDD, AAI, DDI, DDDR, VVIR, DVIR, VDDR, AAIR, DDIR and othermodes of single and dual chamber pacing. In the aforementioned pacingmodes, “D” may indicate dual chamber, “V” may indicate a ventricle, “I”may indicate inhibited pacing (e.g., no pacing), and “A” may indicate anatrium. The first letter in the pacing mode may indicate the chamberthat is paced, the second letter may indicate the chamber in which anelectrical signal is sensed, and the third letter may indicate thechamber in which the response to sensing is provided.

In examples in which IMD 16 is configured to deliver other types ofcardiac rhythm therapy in addition to excitation fusion pacing,intervals defined by pacer timing and control module 92 within processor80 may include atrial and ventricular pacing escape intervals,refractory periods during which sensed P-waves and R-waves areineffective to restart timing of the escape intervals, and the pulsewidths of the pacing pulses. As another example, pacer timing andcontrol module 92 may define a blanking period, and provide signals fromsensing module 86 to blank one or more channels, e.g., amplifiers, for aperiod during and after delivery of electrical stimulation to heart 12.The durations of these intervals may be determined by processor 80 inresponse to stored data in memory 82. The pacer timing and controlmodule of processor 80 may also determine the amplitude of the cardiacpacing pulses.

During pacing modes other than the fusion pacing, escape intervalcounters within pacer timing/control module 92 of processor 80 may bereset upon sensing of R-waves and P-waves. Stimulation generator 84 mayinclude pacer output circuits that are coupled, e.g., selectively by aswitching module, to any combination of electrodes 40, 42, 44, 46, 48,50, 58, 62, or 66 appropriate for delivery of a bipolar or unipolarpacing pulse to one of the chambers of heart 12. Processor 80 may resetthe escape interval counters upon the generation of pacing pulses bystimulation generator 84, and thereby control the basic timing ofcardiac pacing functions, including fusion cardiac resynchronizationtherapy.

The value of the count present in the escape interval counters whenreset by sensed R-waves and P-waves may be used by processor 80 tomeasure the durations of R−R intervals, P−P intervals, P−R intervals andR−P intervals, which are measurements that may be stored in memory 82.Processor 80 may use the count in the interval counters to detect atachyarrhythmia event, such as ventricular fibrillation event orventricular tachycardia event. Upon detecting a threshold number oftachyarrhythmia events, processor 80 may identify the presence of atachyarrhythmia episode, such as a ventricular fibrillation episode, aventricular tachycardia episode, or a non-sustained tachycardia (NST)episode. Examples of tachyarrhythmia episodes that may qualify fordelivery of responsive therapy include a ventricular fibrillationepisode or a ventricular tachyarrhythmia episode.

In some examples, processor 80 may operate as an interrupt drivendevice, and is responsive to interrupts from pacer timing and controlmodule 92, where the interrupts may correspond to the occurrences ofsensed P-waves and R-waves and the generation of cardiac pacing pulses.Any necessary mathematical calculations to be performed by processor 80and any updating of the values or intervals controlled by the pacertiming and control module of processor 80 may take place following suchinterrupts. A portion of memory 82 may be configured as a plurality ofrecirculating buffers, capable of holding series of measured intervals,which may be analyzed by processor 80 in response to the occurrence of apace or sense interrupt to determine whether the patient's heart 12 ispresently exhibiting atrial or ventricular tachyarrhythmia.

In some examples, an arrhythmia detection method may include anysuitable tachyarrhythmia detection algorithms. In one example, processor80 may utilize all or a subset of the rule-based detection methodsdescribed in U.S. Pat. No. 5,545,186 to Olson et al., entitled,“PRIORITIZED RULE BASED METHOD AND APPARATUS FOR DIAGNOSIS AND TREATMENTOF ARRHYTHMIAS,” which issued on Aug. 13, 1996, or in U.S. Pat. No.5,755,736 to Gillberg et al., entitled, “PRIORITIZED RULE BASED METHODAND APPARATUS FOR DIAGNOSIS AND TREATMENT OF ARRHYTHMIAS,” which issuedon May 26, 1998. U.S. Pat. No. 5,545,186 to Olson et al. and U.S. Pat.No. 5,755,736 to Gillberg et al. are incorporated herein by reference intheir entireties. However, other arrhythmia detection methodologies mayalso be employed by processor 80 in other examples.

If IMD 16 is configured to generate and deliver defibrillation pulses toheart 12, stimulation generator 84 may include a high voltage chargecircuit and a high voltage output circuit. In the event that generationof a cardioversion or defibrillation pulse is required, processor 80 mayemploy the escape interval counter to control timing of suchcardioversion and defibrillation pulses, as well as associatedrefractory periods. In response to the detection of atrial orventricular fibrillation or tachyarrhythmia requiring a cardioversionpulse, processor 80 may activate a cardioversion/defibrillation controlmodule (not shown), which may, like pacer timing and control module 92,be a hardware component of processor 80 and/or a firmware or softwaremodule executed by one or more hardware components of processor 80. Thecardioversion/defibrillation control module may initiate charging of thehigh voltage capacitors of the high voltage charge circuit ofstimulation generator 84 under control of a high voltage chargingcontrol line.

Processor 80 may monitor the voltage on the high voltage capacitor,e.g., via a voltage charging and potential (VCAP) line. In response tothe voltage on the high voltage capacitor reaching a predetermined valueset by processor 80, processor 80 may generate a logic signal thatterminates charging. Thereafter, timing of the delivery of thedefibrillation or cardioversion pulse by stimulation generator 84 iscontrolled by the cardioversion/defibrillation control module ofprocessor 80. Following delivery of the fibrillation or tachycardiatherapy, processor 80 may return stimulation generator 84 to a cardiacpacing function and await the next successive interrupt due to pacing orthe occurrence of a sensed atrial or ventricular depolarization.

Stimulation generator 84 may deliver cardioversion or defibrillationpulses with the aid of an output circuit that determines whether amonophasic or biphasic pulse is delivered, whether housing electrode 58serves as cathode or anode, and which electrodes are involved indelivery of the cardioversion or defibrillation pulses. Suchfunctionality may be provided by one or more switches or a switchingmodule of stimulation generator 84.

Telemetry module 88 includes any suitable hardware, firmware, softwareor any combination thereof for communicating with another device, suchas programmer 24 (FIG. 1). Under the control of processor 80, telemetrymodule 88 may receive downlink telemetry from and send uplink telemetryto programmer 24 with the aid of an antenna, which may be internaland/or external. Processor 80 may provide the data to be uplinked toprogrammer 24 and the control signals for the telemetry circuit withintelemetry module 88, e.g., via an address/data bus. In some examples,telemetry module 88 may provide received data to processor 80 via amultiplexer.

In some examples, processor 80 may transmit atrial and ventricular heartsignals (e.g., EGM signals) produced by atrial and ventricular sense ampcircuits within sensing module 86 to programmer 24. Other types ofinformation may also be transmitted to programmer 24, such as thevarious intervals and delays used to deliver the fusion pacing pulse toLV 32. Programmer 24 may interrogate IMD 16 to receive the heartsignals. Processor 80 may store heart signals within memory 82, andretrieve stored heart signals from memory 82. Processor 80 may alsogenerate and store marker codes indicative of different cardiac episodesthat sensing module 86 detects, and transmit the marker codes toprogrammer 24. An example pacemaker with marker-channel capability isdescribed in U.S. Pat. No. 4,374,382 to Markowitz, entitled, “MARKERCHANNEL TELEMETRY SYSTEM FOR A MEDICAL DEVICE,” which issued on Feb. 15,1983 and is incorporated herein by reference in its entirety.

The various components of IMD 16 are coupled to power source 90, whichmay include a rechargeable or non-rechargeable battery. Anon-rechargeable battery may be selected to last for several years,while a rechargeable battery may be inductively charged from an externaldevice, e.g., on a daily or weekly basis.

FIG. 4 is block diagram of an example programmer 24. As shown in FIG. 4,programmer 24 includes processor 100, memory 102, user interface 104,telemetry module 106, and power source 108. Programmer 24 may be adedicated hardware device with dedicated software for programming of IMD16. Alternatively, programmer 24 may be an off-the-shelf computingdevice running an application that enables programmer 24 to program IMD16.

A user may use programmer 24 to select therapy programs (e.g., sets ofstimulation parameters), generate new therapy programs, modify therapyprograms through individual or global adjustments or transmit the newprograms to a medical device, such as IMD 16 (FIG. 1). The clinician mayinteract with programmer 24 via user interface 104, which may includedisplay to present graphical user interface to a user, and a keypad oranother mechanism for receiving input from a user.

Processor 100 can take the form one or more microprocessors, DSPs,ASICs, FPGAs, programmable logic circuitry, or the like, and thefunctions attributed to processor 102 herein may be embodied ashardware, firmware, software or any combination thereof. Memory 102 maystore instructions that cause processor 100 to provide the functionalityascribed to programmer 24 herein, and information used by processor 100to provide the functionality ascribed to programmer 24 herein. Memory102 may include any fixed or removable magnetic, optical, or electricalmedia, such as RAM, ROM, CD-ROM, hard or floppy magnetic disks, EEPROM,or the like. Memory 102 may also include a removable memory portion thatmay be used to provide memory updates or increases in memory capacities.A removable memory may also allow patient data to be easily transferredto another computing device, or to be removed before programmer 24 isused to program therapy for another patient. Memory 102 may also storeinformation that controls therapy delivery by IMD 16, such asstimulation parameter values.

Programmer 24 may communicate wirelessly with IMD 16, such as using RFcommunication or proximal inductive interaction. This wirelesscommunication is possible through the use of telemetry module 106, whichmay be coupled to an internal antenna or an external antenna. Anexternal antenna that is coupled to programmer 24 may correspond to theprogramming head that may be placed over heart 12, as described abovewith reference to FIG. 1.

Telemetry module 106 may be similar to telemetry module 88 of IMD 16(FIG. 3). Telemetry module 106 may also be configured to communicatewith another computing device via wireless communication techniques, ordirect communication through a wired connection. Examples of localwireless communication techniques that may be employed to facilitatecommunication between programmer 24 and another computing device includeRF communication according to the 802.11 or Bluetooth specificationsets, infrared communication, e.g., according to the IrDA standard, orother standard or proprietary telemetry protocols. In this manner, otherexternal devices may be capable of communicating with programmer 24without needing to establish a secure wireless connection.

Power source 108 is configured to deliver operating power to thecomponents of programmer 24. Power source 108 may include a battery anda power generation circuit to produce the operating power. In someembodiments, the battery may be rechargeable to allow extendedoperation. Recharging may be accomplished by electrically coupling powersource 108 to a cradle or plug that is connected to an alternatingcurrent (AC) outlet. In addition or alternatively, recharging may beaccomplished through proximal inductive interaction between an externalcharger and an inductive charging coil within programmer 24. In otherembodiments, traditional batteries (e.g., nickel cadmium or lithium ionbatteries) may be used. In addition, programmer 24 may be directlycoupled to an alternating current outlet to power programmer 24. Powersource 104 may include circuitry to monitor power remaining within abattery. In this manner, user interface 104 may provide a currentbattery level indicator or low battery level indicator when the batteryneeds to be replaced or recharged. In some cases, power source 108 maybe capable of estimating the remaining time of operation using thecurrent battery.

Example techniques that IMD 16 may implement in order to deliveradaptive CRT are described with respect to FIGS. 5-12. FIGS. 5-11illustrate example techniques for selecting a pacing configuration foradaptive CRT based on a surrogate indication of intrinsic AV conductionof heart 12. FIG. 13 illustrates an example technique for adjusting afusion pacing interval based on at least one A_(P/S)−RV_(S) interval.While FIGS. 5-12 are primarily described as being performed by processor80 of IMD 16, in other examples, a processor of another device, such asprocessor 100 of programmer 24, may perform any part of the techniquesdescribed herein, including those described with respect to FIGS. 5-12.

FIG. 5 is a flow diagram of an example technique that IMD 16 mayimplement to provide adaptive CRT to patient 14. When it is determined,e.g., by a clinician, that heart 12 of patient 14 is exhibitingintrinsic conduction, e.g., based on an intrinsic conduction timemeasurement, but one ventricle is contracting out of synchrony with theother, IMD 16 may be configured to generate and deliver fusion pacingtherapy to heart 12 of patient 14. In accordance with the techniqueshown in FIG. 5, processor 80 may control stimulation generator 84 todeliver a fusion pacing pulse to the later contracting ventricle V2,which, in the example described herein, is LV 32 (110). In otherexamples, the later contracting ventricle V2 may be RV 28.

In the example shown in FIG. 5, after delivering a fusion pacing pulse(LV_(P)) to LV 32, processor 80 determines whether a surrogateindication of intrinsic conduction of RV 28 is detected (112). Exampletechniques that processor 80 may implement to determine whether thesurrogate indication is detected described below with respect to FIGS.6-11. In some examples, processor 80 may control the delivery of pacingtherapy to patient 14 based on the detection of a surrogate indicationof intrinsic AV conduction by implementing the technique shown in FIG.5, or any of the other techniques relating to the surrogate indicationdescribed herein (e.g., FIGS. 6-11), if the PEI used to determine afusion pacing interval is greater than the current blanking intervalwith which IMD 16 senses electrical cardiac activity of heart 12following the LV pace event (LV_(P)). Thus, in some examples, prior toimplementing the technique shown in any of FIGS. 5-11, processor 80 maydetermine whether the PEI used to determine the fusion pacing intervalwith which the fusion pacing pulse is delivered to LV (110) is greaterthan the current blanking interval.

If the surrogate indication is not detected following the delivery of afusion pacing pulse (NO branch of block 112), processor 80 may determinethere has been a loss of intrinsic AV conduction from RA 26 to RV 28,such that fusion pacing may be less effective for maintaining adesirable cardiac output of heart 12. Accordingly, in response todetermining that the surrogate indication of intrinsic AV conduction hasnot been detected (112), processor 80 may control stimulation generator84 to switch pacing configurations. In the example shown in FIG. 5,processor 80 controls stimulation generator 84 to terminate fusionpacing and to generate and deliver biventricular pacing to heart 12(114). For example, processor 80 may control stimulation generator 84 togenerate and deliver a first pacing pulse to RV 28 via electrodes oflead 18 and to generate and deliver a second pacing pulse to LV 32 viaelectrodes of lead 20 in a manner that synchronizes contraction of RV 28and LV 28.

Processor 80 may also be configured to, in response to detecting thesurrogate indication of AV conduction (YES branch of block 112),maintain the fusion pacing configuration. For example, processor 80 maycontrol stimulation generator 84 to generate and deliver another fusionpacing pulse to LV 32 (110). Detection of the surrogate indication of AVconduction may indicate that there has not been a loss of intrinsic AVconduction, such that fusion pacing may still be useful for maintaininga desirable level of cardiac output for patient 14. IMD 16 may continuedelivering adaptive CRT to patient 14 using the technique shown in FIG.5. In the example shown in FIG. 5, after delivering each fusion pacingpulse to LV 32 (110), processor 80 may determine whether the surrogateindication of intrinsic AV conduction is detected (112), such thatprocessor 80 determines whether intrinsic AV conduction is present basedon the surrogate indication on a pulse-by-pulse basis.

In other examples, processor 80 may determine whether the surrogateindication of intrinsic RV 28 conduction is detected (112) on a lessfrequent basis, but more frequently than determining the actualintrinsic conduction time measurement (e.g., by suspending delivery ofpacing pulses to LV 32 and RV 28, and determining the time between anatrial pacing or sense event (A_(P/S)) and an RV 28 sense event(RV_(S))). For example, processor 80 may determine whether the surrogateindication of intrinsic RV 28 conduction is detected every other fusionpacing pulse, every three fusion pacing pulses, once a minute, once anhour, or according to another frequency. In some examples, processor 80may determine the surrogate indication (112) and determine whether thesurrogate indication of intrinsic AV conduction indicates a loss ofintrinsic AV conduction (114) more than once during a 24 hour period.

In other examples, rather than immediately switching to biventricularpacing in response to a first failure to detect the surrogate indicationof intrinsic AV conduction after the delivery of a fusion pacing pulseto LV 32, processor 80 may determine whether the surrogate indication ofintrinsic AV conduction is not detected a certain number of times beforeswitching the pacing configuration of IMD 16 to biventricular pacing. Anexample of such a technique is shown in FIG. 6, which is a flow diagramof an example technique that IMD 16 may implement to deliver fusionpacing to heart 12 of patient 14. In the example technique shown in FIG.6, the surrogate indication of intrinsic AV conduction is the detectionof a ventricular activation of RV 28 within a predetermined timeimmediately following the time a pacing stimulus (LV_(P)) to LV 32.

In accordance with the technique shown in FIG. 6, processor 80 (FIG. 3)of IMD 16 controls the delivery of a fusion pacing pulse LV_(P) to LV 32(FIG. 2) relative to an atrial pace or sense event (A_(P/S)). Inparticular, in the example shown in FIG. 6, processor 80 detects theatrial pace or sense event (A_(P/S)) (120), e.g., by detecting a P-wavein an electrical cardiac signal sensed by sensing module 86 (FIG. 3) orby controlling stimulation generator 84 to generate and deliver a pacingstimulus to RA 26. Thereafter, processor 80 may control stimulationgenerator 84 of IMD 16 to deliver a pacing stimulus (LV_(P)) to LV 32(122). For example, after detecting the atrial pace or sense event(A_(P/S)), processor 80 may control the delivery of the LV 32 pacingstimulus (LV_(P)) to heart 12 upon expiration of a fusion pacinginterval stored by memory 82 of IMD 16 or of another device (e.g.,programmer 24). The fusion pacing interval may start at the detection ofthe atrial pace or sense event (A_(P/S)) (120).

After stimulation generator 84 generates and delivers the pacingstimulus (LV_(P)) to LV 32, processor 80 determines whether thesurrogate indication of intrinsic AV conduction is detected. In theexample shown in FIG. 6, processor 80 controls sensing module 86 tosense electrical cardiac activity and determines whether depolarizationof RV 28 (RV_(S)) is detected within a predetermined time windowimmediately following the time the pacing stimulus (LV_(P)) wasdelivered to LV 32 (124). In one example, the predetermined time windowhas a duration of about 30 milliseconds (ms) to about 100 ms and beginsimmediately following the time the pacing stimulus (LV_(P)) wasdelivered to LV 32 (e.g., LV_(P) may be time zero of the predeterminetime window). If the blanking interval with which sensing module 86senses electrical cardiac activity of heart 12 following the LV paceevent (LV_(P)) is greater than about the duration of the predeterminedtime window, processor 80 may shorten the blanking interval in order todetect the surrogate indication of the intrinsic AV conduction.

As discussed in further detail below with reference to FIG. 7, processor80 may detect the surrogate indication of intrinsic AV conduction by notonly determining whether the depolarization of RV 28 (RV_(S)) isdetected within a predetermined time window immediately following thetime the pacing stimulus (LV_(P)) was delivered to LV 32, but alsodetermining whether the depolarization of RV 28 (RV_(S)) wasattributable to the pacing stimulus (LV_(P)) delivered to LV 32. Inthese examples, processor 80 may characterize the RV sense event(RV_(S)) detected within the predetermined time window immediatelyfollowing the time the pacing stimulus (LV_(P)) was delivered to LV 32as the surrogate indication of intrinsic AV conduction if processor 80also determines that the RV sense event (RV_(S)) was not caused by thepacing stimulus (LV_(P)).

In accordance with the technique shown in FIG. 6, in response todetecting the depolarization of RV 28 (RV_(S)) within the predeterminedwindow of time immediately following the delivery of the pacing stimulus(LV_(P)) to LV 32 (124), processor 80 may determine that the surrogateindication of intrinsic AV conduction was detected, and, therefore,there has not been a loss of intrinsic AV conduction. In response,processor 80 may continue controlling stimulation generator 84 togenerate and delivery cardiac rhythm management therapy to heart 12according to a fusion pacing configuration. As shown in FIG. 6 (YESbranch of decision block 124), processor 80 may detect an atrial pace orsense event (A_(P/S)) (120) and, thereafter, control stimulationgenerator 84 to deliver a fusion pacing stimulus (LV_(P)) to LV 32(122). After stimulation generator 84 delivers the pacing stimulus(LV_(P)) to LV 32, processor 80 determines whether the surrogateindication of intrinsic AV conduction is detected (124).

If, however, processor 80 does not detect the surrogate indication ofintrinsic AV conduction after the fusion pacing pulse (LV_(P)) isdelivered to LV 32, processor 80 may increment a counter (126). Thecounter can be implemented by software, hardware, firmware, or anycombination thereof. For example, when processor 80 increments thecounter, processor 80 may generate a flag, value or other indicationgenerated by processor 80 and stored by memory 82 of IMD 16 or a memoryof another device. As another example, the counter may be implemented bya register-type circuit and processor 80 may cause a state of theregister-type circuit to change in order to increment or otherwisemanage the counter. Counters having other configurations may also beused. In the example shown in FIG. 6, processor 80 determines thesurrogate indication of intrinsic AV conduction is not detected inresponse to determining depolarization of RV 28 (RV_(S)) is not detectedwithin a predetermined time window immediately following the time thepacing stimulus (LV_(P)) was delivered to LV 32 (NO branch of block124).

In some examples, processor 80 only increments the counter for eachconsecutive cardiac cycle in which the surrogate indication of intrinsicAV conduction is not detected. In these examples, processor 80 may resetthe counter to zero each time the surrogate indication of intrinsic AVconduction is detected. In other examples, processor 80 increments thecounter for nonconsecutive cardiac cycle in which the surrogateindication of intrinsic AV conduction is not detected, and resets thecounter at other times, e.g., if the surrogate indication is detectedfor two or more consecutive cardiac cycles.

After processor 80 increments the counter (126), processor 80 maydetermine whether the value of the counter is greater than apredetermined counter threshold value (128). In some examples, thepredetermined counter threshold value is two, while in other examples,the predetermined counter threshold value is one, three, four or more.The predetermined counter threshold value may be value determined by aclinician to be indicative of a loss of intrinsic AV conduction, and maybe selected to be low enough to configure IMD 16 to provide a responsiveswitch in pacing configuration, and to provide responsive cardiac rhythmmanagement therapy. The predetermined counter threshold value may bestored by memory 82 of IMD 16 or a memory of another device with whichprocessor 80 may communicate (e.g., programmer 24).

In some examples, processor 80 restarts the counter each time asurrogate indication of intrinsic AV conduction is detected. In theseexamples, the counter tracks consecutive, uninterrupted failures todetect the surrogate indication of intrinsic AV conduction. In otherexamples, processor 80 manages the counter to track the number offailures to detect the surrogate indication of intrinsic AV conductionwithin a predetermined range of time (e.g., within 30 seconds, oneminute or more).

If processor 80 determines that the value of the counter is not greaterthan the predetermined threshold value (NO branch of block 128),processor 80 may, in response, resume control of fusion pacing topatient 14, e.g., by restarting the technique shown in FIG. 6. Forexample, processor 80 may detect an atrial pace or sense event (A_(P/S))(120), subsequently control stimulation generator 84 of IMD 16 todeliver a pacing stimulus (LV_(P)) to LV 32 (122), and determine whetherthe depolarization of RV 28 (RV_(S)) within the predetermined window oftime immediately following the delivery of the pacing stimulus (LV_(P))to LV 32 (124).

If, however, after processor 80 increments the counter (126), processor80 determines the value of the counter is greater than the predeterminedthreshold value (Yes branch of block 128), processor 80 may terminatefusion pacing and control stimulation generator 84 to deliverbiventricular pacing to heart 12 of patient 14 (116). A value of thecounter is greater than the predetermined threshold value may indicatethat the intrinsic AV conduction is no longer present or that a changein pacing configuration to better suit the patient's physiological stateis desirable.

In other examples of the techniques shown in FIGS. 5 and 6, rather thanterminating fusion pacing and controlling stimulation generator 84 todeliver biventricular pacing to heart 12 of patient 14 in response todetermining the value of the counter is greater than the predeterminedthreshold value, processor 80 may control IMD 16 to perform an intrinsicAV conduction time measurement. In one example, processor 80 performs anintrinsic AV conduction time measurement by suspending all cardiacrhythm therapy to RV 28 and LV 32 and determining the time delay betweenan atrial pace or sense event (A_(P/S)) and an RV sense event (RV_(S))for one or more cardiac cycles. If the measured intrinsic AV conductiontimes from a plurality of cardiac cycle are used to determine theintrinsic AV conduction time measurement, processor 80 may, for example,determine the intrinsic AV conduction time is the mean or median of theplurality of measured AV conduction times for the cardiac cycles.

In some examples, processor 80 determines the intrinsic conduction timemeasurement indicates a loss of intrinsic AV conduction if the intrinsicconduction time measurement is greater than (or greater than or equalto) a predetermined threshold value (which differs from the counterthreshold discussed above). The predetermined threshold value may beselected to be, for example, a value that indicates the depolarizationof RV 28 is delayed to such an extent that cardiac output of heart 12 ofpatient 14 may not be physiologically sufficient. If the intrinsicconduction time measurement indicates a loss of intrinsic AV conduction,processor 80 may control stimulation generator 84 to terminate fusionpacing and deliver biventricular pacing to heart 12 of patient 14.

As described above, in some examples, sensing module 86 may sensedepolarization of RV 28 (RV_(S)) after delivery of a fusion pacing pulse(LV_(P)) to LV 32 even if intrinsic conduction from RA 26 to RV 28 isnot actually present. For example, the electrical stimulus delivered toLV 32 may propagate to RV 26 and thereby cause depolarization of RV 26.In these situations, detection of the depolarization of RV 28 that isattributable to the pacing stimulus (LV_(P)) delivered to LV 32 may notbe a proper surrogate indication of intrinsic AV conduction. In someexamples, as part of the technique for detecting the surrogateindication of intrinsic AV conduction, processor 80 of IMD 16 maydetermine whether depolarization of RV 28 (RV_(S)) sensed after deliveryof a pacing stimulus (LV_(P)) to LV 32 is attributable to the pacingstimulus (LV_(P)).

FIG. 7 is a flow diagram of an example technique for determining whetherdepolarization of RV 28 (RV_(S)) sensed after delivery of a pacingstimulus (LV_(P)) to LV 32 is attributable to the pacing stimulus(LV_(P)) or to intrinsic AV conduction. In the technique shown in FIG.7, processor 80 controls the stimulation generator 84 to deliver apacing pulse (LV_(P)) to LV 32 (FIG. 2) at a time that is based on anatrial pace or sense event (A_(P/S)). In particular, in the exampleshown in FIG. 7, processor 80 detects the atrial pace or sense event(A_(P/S)) (120), and controls stimulation generator 84 of IMD 16 todeliver a pacing stimulus (LV_(P)) to LV 32 (122) after expiration of afusion pacing interval that begins at the detection of the atrial paceor sense event (A_(P/S)). After stimulation generator 84 generates anddelivers the pacing stimulus (LV_(P)) to LV 32, processor 80 determineswhether the surrogate indication of intrinsic AV conduction is detected(124). In the example shown in FIG. 7, the surrogate indication isdepolarization of RV 28 (RV_(S)) within a predetermined time windowimmediately following the delivery of the pacing stimulus (LV_(P)) to LV32.

If processor 80 does not detect the surrogate indication of intrinsic AVconduction (NO branch of block 124 in FIG. 7), processor 80 mayincrement a counter (126), determine whether the value of the counter isgreater than a predetermined threshold value (128), and, if so,terminate fusion pacing and deliver biventricular pacing (116) or takeanother action, as described with respect to FIG. 6. As also describedwith respect to FIG. 6, in response to determining the value of thecounter is not greater than the predetermined threshold value (NO branchof block 128), processor 80 may control stimulation generator 84 tomaintain fusion pacing therapy delivery to LV 32.

In response to detecting the surrogate indication of intrinsic AVconduction (YES branch of block 124 in FIG. 7), processor 80 maydetermine the A_(P/S)−RV_(S) interval, e.g., by determining the durationof time between the atrial pace or sense event (A_(P/S)) and the senseddepolarization of RV 28 (RV_(S)) (130). Thus, the A_(P/S)−RV_(S)interval may indicate the time delay, within a common cardiac cycle,between the atrial activation and the depolarization of RV 28. Processor80 may then determine whether the A_(P/S)−RV_(S) interval indicatesintrinsic AV conduction is present (132). For example, as describedbelow with respect to FIG. 9, processor 80 may determine whether theA_(P/S)−RV_(S) interval is approximately equal (e.g., equal or within apredetermined range) to the most recent intrinsic conduction timemeasurement, a mean or median of a plurality of the most recentintrinsic conduction time measurements, a greatest intrinsic conductiontime measurement of a plurality of the most recent intrinsic conductiontime measurements, or a smallest intrinsic conduction time measurementof a plurality of the most recent intrinsic conduction timemeasurements. In some examples, processor 80 determines whether theA_(P/S)−RV_(S) interval is approximately equal to a selected one ofthese values, such as equal to or within a predetermined percentage(e.g., about 95% to about 105%, such as about 100%) of the selectedvalue. The recent intrinsic conduction time measurement or a pluralityof the most recent intrinsic conduction time measurements can be storedby memory 82 of IMD 16 or a memory of another device, such as programmer24. In response to determining that A_(P/S)−RV_(S) interval isapproximately equal to the selected value that is based on the one ormore recent intrinsic conduction time measurements, processor 80 maydetermine that the A_(P/S)−RV_(S) interval is indicative of the presenceof intrinsic conduction of heart 12.

In another example, as described in further detail below with referenceto FIG. 8, instead of, or in addition to, determining whether theA_(P/S)−RV_(S) interval is approximately equal to the most recentintrinsic conduction time measurement in order to determine whether theA_(P/S)−RV_(S) interval is indicative of intrinsic AV conduction,processor 80 determines whether the interval of time between the LV 32pacing stimulus (LV_(P)) and the RV_(S) sense event detected within thepredetermined window of time immediately following the LV pacingstimulus (the interval being referred to as an “LV_(P)−RV_(S) interval”)is less than a duration of time between pacing pulse delivered to LV 32earlier in time (LV_(EP)) following the atrial pacing or sense event(A_(P/S)) than the fusion pacing stimulus (LV_(P)) and a subsequent RV28 sense event (RV_(S, EP)). In response to determining theLV_(P)−RV_(S) interval is less than such a duration of time(LV_(EP)−RV_(S, EP)), processor 80 may determine that the A_(P/S)−RV_(S)interval is indicative of the presence of intrinsic conduction. In someexamples, processor 80 determines that the A_(P/S)−RV_(S) interval isindicative of the presence of intrinsic conduction of heart 12 inresponse to determining that the LV_(P)−RV_(S) interval is less such theduration (LV_(EP)−RV_(S, EP)) and the A_(P/S)−RV_(S) interval isapproximately equal to a value that is based on the one or more mostrecent intrinsic conduction time measurements.

In the technique shown in FIG. 7, in response to determining that theA_(P/S)−RV_(S) interval is not indicative of intrinsic AV conduction (NObranch of block 132), processor 80 determines that the surrogateindication of intrinsic AV conduction is not, in fact, indicative ofintrinsic AV conduction and that the depolarization of RV 28 (RV_(S))sensed within the predetermined window of time immediately following thepacing stimulus delivered to LV 32 (LV_(P)) was attributable to (e.g.,caused by) the pacing stimulus delivered to LV 32 (LV_(P)). In response,processor 80 may increment a counter (126), determine whether the valueof the counter is greater than a predetermined threshold value (128),and, take the appropriate action based on whether the value of thecounter is greater than a predetermined threshold value. For example,processor 80 may terminate fusion pacing and deliver biventricularpacing (116) or perform an intrinsic conduction measurement if the valueof the counter is greater than a predetermined threshold value, asdescribed with respect to FIG. 6. In addition, as described with respectto FIG. 6, if the value of the counter is not greater than thepredetermined threshold value (NO branch of block 128), processor 80 mayresume fusion pacing (120, 122) and, in some examples, restart thetechnique shown in FIG. 7.

If processor 80 determines that the A_(P/S)−RV_(S) interval isindicative of intrinsic AV conduction (YES branch of block 132),processor 80 determines that RV sense event (RV_(S)) detected within thepredetermined window of time following the fusion pacing pulse (LV_(P))is a surrogate indication of intrinsic AV conduction. In response,processor 80 may control stimulation generator 84 to maintain fusionpacing therapy to heart 12 (120, 122), and, in some examples, restartthe technique shown in FIG. 7.

FIG. 8 is a flow diagram of an example technique that may beimplemented, e.g., by processor 80, to determine whether an interval oftime (A_(P/S)−RV_(S) interval) between an atrial pace or sense event(A_(P/S)) and an RV 28 sense event (RV_(S)) sensed within apredetermined window of time immediately following the LV pacingstimulus (LV_(P)) is indicative of intrinsic AV conduction. In someexamples, the A_(P/S)−RV_(S) interval may be determined using thetechnique shown in FIG. 7. In accordance with the technique shown inFIG. 8, processor 80 controls stimulation generator 84 to deliver apacing pulse to LV 32 (LV_(EP)) at a time prior to the time indicated bythe fusion pacing interval used to deliver fusion pacing therapy toheart 12 (134). Thus, processor 80 may effectively cause the fusionpacing interval to be temporarily shortened, such that the LV pacingstimulus is delivered closer in time to the atrial pace or sense eventof the same cardiac cycle. If the fusion pacing interval is determinedusing Equation (1), for example, processor 80 may increase the PEI inorder to decrease the fusion pacing interval. The pacing intervalindicates the time at which stimulation generator 84 delivers the pacingpulse to LV 32 relative to an atrial pace or sense event (A_(P/S)). Thepacing interval for the early pacing pulse to LV 32 (LV_(EP)) isselected to be a duration of time that is short enough to time theearlier pacing pulse to LV 32 (LV_(EP)) (relative to an atrial pace orsense event (A_(P/S))) to be before the intrinsic depolarization of RV28. In some examples, processor 80 sets the pacing interval for theearlier pacing pulse to LV 32 (LV_(EP)) to be equal to about zero. Inother examples processor 80 sets the pacing interval for the earlierpacing pulse to LV 32 (LV_(EP)) by decrementing a stored fusion pacinginterval by a predetermined amount, such as about 150 ms.

After stimulation generator 84 delivers the early pacing pulse to LV 32(LV_(EP)), sensing module 84 may sense depolarization of RV 28(RV_(S, EP)) (136) and processor 80 may determine the time delay(LV_(EP)−RV_(S, EP)) between the early pacing pulse to LV 32 (LV_(E))and the subsequently detected RV 28 sense event (RV_(S, EP)) (138). Insome examples, processor 80 determines the LV_(EP)−RV_(S, EP) interval(138) less frequently than determining the A_(P/S)−RV_(S) andLV_(P)−RV_(S) intervals, such as about once a day. For example,processor 80 may control stimulation generator 84 to deliver the earlypacing pulse to LV 32 (LV_(EP)) only once a day and may store thedetermined LV_(EP)−RV_(S, EP) in memory 82 of IMD 16 or a memory ofanother device (e.g., a programmer).

According to the technique shown in FIG. 8, processor 80 may determinewhether the LV_(EP)−RV_(S, EP) interval of the cardiac cycle includingthe early pacing pulse to LV 32 (LV_(EP)) is greater than (or, in someexamples, greater than or equal to) the LV_(P)−RV_(S) interval(determined for a cardiac cycle including the normal fusion pacinginterval). Processor may determine the LV_(P)−RV_(S) interval by, forexample, determining (e.g., using the same cardiac cycle for which theA_(P/S)−RV_(S) interval was determined) the interval of time between theLV 32 fusion pacing pulse (LV_(P)) and the RV sense event (RV_(S))sensed within the predetermined window of time immediately following theLV 32 fusion pacing pulse (LV_(P)). Thus, the LV_(P)−RV_(S) interval isless than the A_(P/S)−RV_(S) interval. The LV_(P)−RV_(S) interval mayindicate the conduction time without any interference from atrialconduction.

If the LV_(EP)−RV_(S, EP) interval is greater than the LV_(P)−RV_(S)interval (YES branch of block 140), it may indicate that RV 28 senseevent (RV_(S)) sensed within the predetermined window of timeimmediately following the LV 32 pacing pulse (LV_(P)) occurred due tointrinsic AV conduction to RV 28 and not due to propagation ofelectrical stimulation from the pacing stimulus delivered to LV 32(LV_(P)) to RV 28. Accordingly, in response determining theLV_(EP)−RV_(S, EP) interval is greater than the LV_(P)−RV_(S) interval(YES branch of block 140), processor 80 may determine that the RV senseevent (RV_(S)) is attributable to intrinsic conduction (142). This mayalso correspond to, for example, the YES branch of block 132 in FIG. 7.

In some examples, processor 80 may generate a surrogate indication ofintrinsic conduction (144) in response to determining that the RV senseevent (RV_(S)) used to determine the LV_(P)−RV_(S) interval and theA_(P/S)−RV_(S) interval is attributable to intrinsic conduction (142).The surrogate indication of intrinsic conduction, when generated,indicates that heart 12 of patient 14 is intrinsically conducting, atleast from RA 26 to RV 28. As a result, in response to generating thesurrogate indication of intrinsic conduction, processor 80 may continuecontrolling stimulation generator 84 to generate and deliver fusionpacing therapy to heart 12. In some examples, processor 80 generates anindication (144), such as a flag, value, or the like, that correspondsto the surrogate indication of intrinsic conduction, and stores theindication in memory 82 or a memory of another device, such asprogrammer 24.

In the technique shown in FIG. 8, processor 80 controls stimulationgenerator 84 to deliver a pacing pulse to LV 32 earlier in time relativeto the atrial pace or sense event (A_(P/S)) compared to the timing ofthe fusion pacing pulse delivered to LV 32 that was used to determinethe LV_(EP)−RV_(S, EP) interval in order to determine whether the pacingstimulus delivered to LV 32 (LV_(P)) is affecting the contraction of RV28. In some patients, when intrinsic AV conduction is present and allother physiological conditions are equal (e.g., the patient'srespiration rate is the same), RV 28 depolarizes at approximately thesame time relative to the atrial pace or sense event (A_(P/S)), despitethe delivery of the pacing stimulus delivered LV 32 (LV_(P)). Byshifting the timing of the pacing pulse to LV 32 to be earlier in timerelative to the atrial pace or sense event (A_(P/S)), processor 80 candetermine, based on the timing of the RV sense event (RV_(S)), whetherthe pacing stimulus is affecting (e.g., evoked) the depolarization of(RV_(S)) by determining whether the RV sense event (RV_(S)) was alsoearlier in time or occurred at approximately the same time. The RV senseevent (RVs) occurring at approximately the same time or at least notearlier in time may be indicated by, for example, the LV_(EP)−RV_(S, EP)interval being greater than the LV_(P)−RV_(S) interval.

In the example shown in FIG. 8, if processor determines that theLV_(EP)−RV_(S, EP) interval of the cardiac cycle including the earlypacing pulse to LV 32 (LV_(EP)) is not greater than the LV_(P)−RV_(S)interval (NO branch of block 140), processor 80 may determine that thedepolarization of RV 28 indicated by the RV sense event (RV_(S)) was atleast partially attributable to the pacing stimulus delivered to LV 32(LV_(P)) (146). Processor 80 may, accordingly, determine that nosurrogate indication of intrinsic AV conduction was detected based onthe RV sense event (RV_(S)). In response to determining that theLV_(EP)−RV_(S, EP) is not greater than the LV_(P)−RV_(S) interval,processor 80 may generate an indication of a failure to detect thesurrogate indication of intrinsic AV conduction (148). In some examples,processor 80 generates the indication of the failure to detect thesurrogate indication (148) by at least generating a flag, value, or thelike that corresponds to the indication of the failure to detect thesurrogate indication, and storing the indication in memory 82 or amemory of another device, such as programmer 24.

The generation of the indication of a failure to detect the surrogateindication of intrinsic AV conduction (148) may correspond to the NObranch of block 132 in FIG. 7. As shown in FIG. 7, in response todetermining there is no indication of intrinsic AV conduction, processor80 may increment a counter (126), determine whether the counter value isgreater than a predetermined threshold value (128), and take anappropriate action based on the determination (116, 120).

As discussed above, in some examples, processor 80 of IMD 16 may confirmthat the RV 28 sense event (RV_(S)) detected within a predeterminedwindow of time following the delivery of a fusion pacing pulse (LV_(P))is indicative of intrinsic AV conduction based on a comparison of thetime interval between the atrial pace or sense event (A_(P/S)) and theRV 28 sense event (RV_(S)) (A_(P/S)−RV_(S)) to an intrinsic conductiontime measurement. The RV 28 sense event (RV_(S)) used (e.g., byprocessor 80) to determine the A_(P)−RV_(S) interval is the same as theRV 28 sense event used (e.g., by processor 80) to determine theLV_(P)−RV_(S) discussed with respect to FIG. 8.

FIG. 9 is a flow diagram of an example technique that may be implementedto determine, based on such a comparison between the A_(P/S)−RV_(S)interval and one or more intrinsic conduction time measurements, whetherthe RV 28 sense event (RV_(S)) was attributable to intrinsic AVconduction.

In accordance with the technique shown in FIG. 9, processor 80 maydetermine an intrinsic conduction time measurement(A_(P/S)−RV_(S, INTRINSIC)) (150). The intrinsic conduction timemeasurement may be, for example, part of a capture management testmeasurement performed by processor 80. In some examples, processor 80determines the intrinsic conduction time measurement by suspending atleast the pacing therapy delivered to RV 28 and LV 32 to allow the heartof the patient to conduct in the absence of cardiac rhythm managementtherapy. In some examples, however, pacing to RA 26 or LA 33 may bemaintained. Processor 80 may then detect, based on electrical cardiacactivity sensed by sensing module 86, a RV 28 sense event (RV_(S)), and,in some cases, an RA sense event. Processor 80 may determine themeasurement of intrinsic conduction time to be the time between anatrial pace or sense event (A_(P/S)) and the RV sensing event (RV_(S)).In some examples, processor 80 stores the intrinsic conduction timemeasurement in memory 82 of IMD 16 or a memory of another device.

After processor 80 determines the intrinsic conduction time measurement(A_(P/S)−RV_(S, INTRINSIC)) (150), processor 80 may determine whetherthe A_(P/S)−RV_(S) interval is approximately equal to the intrinsicconduction time measurement (A_(P/S)−RV_(S, INTRINSIC)) (152). Forexample, processor 80 may determine whether the A_(P/S)−RV_(S) intervalis within a predetermined range of the intrinsic conduction timemeasurement (A_(P/S)−RV_(S, INTRINSIC)), such as equal to or within apredetermined percentage (e.g., about 5% to about 15%, such as about10%) of the intrinsic conduction time measurement(A_(P/S)−RV_(S, INTRINSIC)). In other examples, processor 80 maydetermine whether the A_(P/S)−RV_(S) interval is approximately equal toa mean or median of a plurality of the most recent intrinsic conductiontime measurements, a greatest intrinsic conduction time measurement of aplurality of the most recent intrinsic conduction time measurements, ora smallest intrinsic conduction time measurement of a plurality of themost recent intrinsic conduction time measurements.

In response to determining the A_(P/S)−RV_(S) interval is approximatelyequal to the intrinsic conduction time measurement(A_(P/S)−RV_(S, INTRINSIC)) (YES branch of block 152), processor 80 maydetermine that the RV sense event (RV_(S)) used to determine theA_(P/S)−RV_(S) interval is indicative of the presence of intrinsicconduction of heart 12 (142) and may generate a surrogate indication ofintrinsic conduction (144), as described above with respect to FIG. 8.

On the other hand, in response to determining that the A_(P/S)−RV_(S)interval is not approximately equal to the intrinsic conduction timemeasurement (A_(P/S)−RV_(S, INTRINSIC)) (NO branch of block 152),processor 80 may determine that the RV sense event (RV_(S)) used todetermine the A_(P/S)−RV_(S) interval is not indicative of the presenceof intrinsic conduction of heart 12 (146) and may generate an indicationof a failure to detect the surrogate indication (148), as describedabove with respect to FIG. 8.

FIG. 10 is a flow diagram of an example technique that may beimplemented to determine whether a pattern of determined A_(P/S)−RV_(S)intervals over time may indicate a loss of intrinsic AV conduction.Processor 80 may implement the technique shown in FIG. 10 to, forexample, determine whether the A_(P/S)−RV_(S) intervals indicate achange in pacing configuration from a fusion pacing configuration to abiventricular pacing configuration is desirable, or to determine whetherto perform an intrinsic conduction time measurement to confirm thepresence of intrinsic AV conduction.

As shown in FIG. 10, processor 80 may detect an atrial pace or senseevent (A_(P/S)) (120) and, thereafter, control stimulation generator 84to deliver a fusion pacing stimulus (LV_(P)) to LV 32, e.g., using afusion pacing interval that is timed relative to the atrial pace orsense event (A_(P/S)) (122). After stimulation generator 84 delivers thepacing stimulus (LV_(P)) to LV 32, processor 80 may determine whethersensing module 86 sensed depolarization of RV 28 (RV_(S)) within apredetermined time window immediately following the time the pacingstimulus (LV_(P)) was delivered to LV 32 (124).

As discussed with respect to FIG. 7, in response to determining that thedepolarization of RV 28 (RV_(S)) was not sensed within a predeterminedtime window immediately following the delivery of the pacing stimulus(LV_(P)) (NO branch of block 124), processor 80 may determine that thesurrogate indication of intrinsic AV conduction was not detected. Inresponse, processor 80 may increment a counter (126), and determinewhether the counter value is greater than (or, in some examples, greaterthan or equal to) a threshold value (128). If the counter value isgreater than the threshold value (YES branch of block 128), processor 80may switch from a fusion pacing configuration to a biventricular pacingconfiguration (116), as shown in FIG. 10, or may suspend fusion pacingand perform an intrinsic AV conduction time measurement. If the countervalue is not greater than the threshold value (NO branch of block 128),processor 80 may resume fusion pacing (120, 122), as shown in FIG. 10.

In response to determining the depolarization of RV 28 (RV_(S)) wassensed within a predetermined time window immediately following thedelivery of the pacing stimulus (LV_(P)) (YES branch of block 124),processor 80 may determine the A_(P/S)−RV_(S) interval (130). In thetechnique shown in FIG. 10, processor 80 determines whether THEA_(P/S)−RVs interval is increasing over time (154). In some examples,processor 80 makes this determination based on the plurality ofA_(P/S)−RV_(S) intervals (of respective cardiac cycles) determinedwithin a particular time frame (e.g., the past 30-60 minutes or aspecified number of most recent A_(P/S)−RV_(S) interval determinations,such as the 10-50 most recent A_(P/S)−RV_(S) intervals).

Processor 80 may determine the A_(P/S)−RV_(S) interval is increasingover time (154) using any suitable technique. In one example, processor80 determines the A_(P/S)−RV_(S) interval is increasing over time if acertain number of consecutive A_(P/S)−RV_(S) intervals (e.g., one, two,three, or more) have increased relative to the prior A_(P/S)−RV_(S)interval determinations, e.g., by a threshold amount (e.g., 1 ms toabout 5 ms). As another example, processor 80 may determine theA_(P/S)−RV_(S) interval is increasing over time if the average of theA_(P/S)−RV_(S) intervals for a particular time frame has increased,e.g., by a threshold amount (e.g., 1 ms to about 5 ms), relative to theaverage of A_(P/S)−RV_(S) interval for the immediate prior time frame.In another example, processor 80 may determine the A_(P/S)−RV_(S)interval is increasing over time if the difference between theA_(P/S)−RV_(S) interval of the most recent cardiac cycle (or ashort-term average) and a longer-term average is greater than or equalto a predetermined number, which may be the cumulative sum of thedifferences or a fixed value. Other techniques may be used to determinewhether the A_(P/S)−RV_(S) interval is increasing over time.

In response to determining the A_(P/S)−RV_(S) interval (for respectivecardiac cycles) is increasing over time (YES branch of block 154),processor 80 may determine that there may be a loss of intrinsic AVconduction, such that the RV 28 sense event (RV_(S)) is not indicativeof intrinsic AV conduction. Thus, in response to determining that theA_(P/S)−RV_(S) interval is increasing over time (YES branch of block154), processor 80 may increment a counter (126), and determine whetherthe counter value is greater than (or, in some examples, greater than orequal to) a threshold value (128). Processor 80 may take the appropriateaction upon determining whether the counter value is greater than thethreshold value (116, 120).

In response to determining that the A_(P/S)−RV_(S) interval is notincreasing over time (NO branch of block 154), processor 80 maydetermine that the right ventricular sense event (RV_(S)) is indicativeof intrinsic AV conduction, such that the surrogate indication ofintrinsic AV conduction is detected. Processor 80 may then maintain thefusion pacing configuration (120, 122), as shown in FIG. 10.

Other techniques in addition to, or instead of, the techniques describedwith respect to FIGS. 7-10 may be used to determine whether an RV senseevent (RV_(S)) detected within a predetermined time window immediatelyfollowing the time the pacing stimulus (LV_(P)) was delivered to LV 32was attributable to intrinsic conduction or to paced activation from theLV 32 pacing stimulus (LV_(P)), and, therefore, whether the RV senseevent (RV_(S)) may be a surrogate indication of the presence ofintrinsic conduction of heart 12. In some examples, processor 80determines whether the RV sense event (RV_(S)) detected within apredetermined time window following the time the pacing stimulus(LV_(P)) was delivered to LV 32 was attributable to intrinsic conductionbased on an amplitude of the electrical cardiac signal used to detectthe RV sense event. The amplitude of the electrical cardiac signalsensed within the predetermined time window immediately following thetime the pacing stimulus (LV_(P)) was delivered to LV 32 may differdepending on whether RV 28 depolarized due to a paced activation fromthe LV 32 pacing stimulus (LV_(P)) or due to intrinsic AV conduction.While the amplitude of the R-wave of an EGM is primarily referred toherein, in other examples, other waves of an EGM or other electricalcardiac signals may be used to determine whether the RV sense event(RV_(S)) was attributable to intrinsic conduction.

FIG. 11 is a flow diagram of another example technique for determiningwhether depolarization of RV 28 (RV_(S)) sensed after delivery of apacing stimulus (LV_(P)) to LV 32 is attributable to the pacing stimulus(LV_(P)). In accordance with the technique shown in FIG. 11, processor80 detects an atrial pace or sense event (A_(P/S)) (120), and controlsstimulation generator 84 of IMD 16 to deliver a pacing stimulus (LV_(P))to LV 32 (122) after expiration of a fusion pacing interval that beginsat the detection of the atrial pace or sense event (A_(P/S)). Afterstimulation generator 84 generates and delivers the pacing stimulus(LV_(P)) to LV 32, processor 80 determines whether an R-wave of an EGM(indicating activity of heart 12) is detected within a predeterminedtime window immediately following the delivery of the pacing stimulus(LV_(P)) to LV 32 (156). The detection of the R-wave may indicatedepolarization of RV 28 (RV_(S)).

If processor 80 does not detect the R-wave of the EGM within thepredetermined window of time (NO branch of block 156 in FIG. 11),processor 80 may increment a counter (126), determine whether the valueof the counter is greater than a predetermined threshold value (128),and, if so, terminate fusion pacing and deliver biventricular pacing(116) or take another action, as described with respect to FIG. 6. Asalso described with respect to FIG. 6, in response to determining thevalue of the counter is not greater than the predetermined thresholdvalue (NO branch of block 128), processor 80 may control stimulationgenerator 84 to maintain fusion pacing therapy delivery to LV 32.

In the technique shown in FIG. 11, in response to detecting the R-wavewithin the predetermined window of time (YES branch of block 156 in FIG.7), processor 80 determines the amplitude of the R-wave (157). Processor80 may then determine whether the amplitude of the R-wave is within apredetermined range of a baseline amplitude value (158). The amplitudeof the R-wave of an electrical cardiac signal sensed within RV 28 maydiffer depending on whether RV 28 depolarized due to a paced activationfrom the LV 32 pacing stimulus (LV_(P)) or due to intrinsic AVconduction.

FIG. 12 is a table that includes data that indicates the amplitude of anR-wave may change depending on whether the R-wave was caused byintrinsic AV conduction or by an LV pacing pulse. FIG. 12 illustrates,for 24 human subjects, the peak R-wave amplitude of an EGM when theR-wave is known to be attributable to a pacing pulse (labeled in FIG. 12as “LV−RV Conduction Peak R-Wave EGM amplitude”). These R-wave amplitudevalues were determined based on an EGM sensed during an LV capturemanagement test. FIG. 12 also illustrates, for the same 24 humansubjects, the peak R-wave amplitude of an EGM when the RV depolarizeddue to intrinsic conduction (referred to in FIG. 12 as “AV ConductionPeak R-Wave EGM amplitude”). These R-wave amplitude values weredetermined based on an EGM sensed during an intrinsic conduction timemeasurement determination. The values indicated a “BLOCK” indicate thatAV-block was detected.

As shown in FIG. 12, the “LV−RV Conduction Peak R-Wave EGM amplitude”value for each subject differs from the “AV Conduction Peak R-Wave EGMamplitude” value, such that the amplitude value of an R-wave mayindicate whether RV depolarized due to intrinsic AV conduction or due topaced activation.

Returning now to FIG. 11, the baseline amplitude value may be, forexample, a predetermined amplitude value for an R-wave that indicates RV28 depolarized due to intrinsic AV conduction. In some examples, thebaseline amplitude value is predetermined, e.g., by a clinician, andstored by memory 82 of IMD 16 or a memory of another device (e.g.,programmer 24). In other examples, processor 80 automatically determinesthe baseline amplitude value, e.g., based on the electrical cardiacsignal sensed in RV 28 as part of an intrinsic conduction timemeasurement determination (e.g., when no pacing is delivered to LV 32)or based on an electrical cardiac signal otherwise known to be sensedwhen intrinsic AV conduction is present in heart 12.

The predetermined range of the baseline amplitude value may also bestored by memory 82 of IMD 16 or a memory of another device (e.g.,programmer 24). The predetermined range of the baseline amplitude valuemay indicate, for example, the variance of the R-wave amplitude valuerelative to the baseline amplitude value that indicates the R-wave isattributable to intrinsic AV conduction. In some examples, thepredetermined range may be, for example, within about 2 millivolts (mV)(e.g., 2 mV or nearly 2 mV) of the baseline amplitude value. In otherexamples, the predetermined range may be a percentage of the baselineamplitude value, such as within about 5% of the baseline amplitudevalue. Other predetermined ranges may be used.

In response to determining the amplitude of the R-wave is within thepredetermined range of the baseline amplitude value (YES branch of block158), processor 80 may determine that the RV sense event (RV_(S)) isindicative of intrinsic AV conduction. In the example shown in FIG. 11,in response to determining the amplitude of the R-wave is within thepredetermined range of the baseline amplitude value, processor controlsstimulation generator 84 to deliver a fusion pacing stimulus to heart 12for the next cardiac cycle (120, 122). Processor 80 may, in someexamples, repeat the technique shown in FIG. 11 for the next cardiaccycle.

On the other hand, in response to determining the amplitude of theR-wave is within the predetermined range of the baseline amplitudevalue, processor 80 may determine that the RV sense event (RV_(S)) isnot a surrogate indication of intrinsic AV conduction (NO branch ofblock 158), and may increment counter (126), determine whether the valueof the counter is greater than a predetermined threshold value (128),and so forth.

The technique shown in FIGS. 7-11 can also be used in combination witheach other in some examples. For example, if any one or more of thecomparison of a determined A_(P/S)−RV_(S) interval to a predeterminedmeasurement of intrinsic conduction time (e.g., as described withrespect to FIG. 7 and FIG. 9), a comparison of the LV_(P)−RV_(S)interval to a time delay between an early pacing pulse to LV 32(LV_(EP)) and a subsequent RV 28 sense event (RV_(S, EP)) (e.g., asdescribed with respect to FIG. 8), a pattern of determinedA_(P/S)−RV_(S) intervals over time (as described with respect to FIG.10), an amplitude of an R-wave sensed within the predetermined window oftime, indicates the RV sense event (RV_(S)) detected within apredetermined window of time after the delivery of a fusion pacingstimulus to LV 32 (LV_(P)) is not attributable to intrinsic AVconduction or otherwise indicates a loss of intrinsic AV conduction,processor 80 may take a responsive action. Example responsive actionsinclude, for example, determining an intrinsic A−RV conduction timemeasurement by suspending all stimulation delivery to heart 12 orsuspending delivery of fusion pacing to heart 12 and initiating deliveryof biventricular pacing to heart 12.

FIG. 13 is a flow diagram of an example technique that may beimplemented to adjust a fusion pacing interval based on anA_(P/S)−RV_(S) interval determined using the RV sense event (RV_(S))detected within a predetermined window of time after the delivery of afusion pacing stimulus to LV 32 (LV_(P)). In the technique shown in FIG.13, processor 80 controls stimulation generator 84 to generate anddeliver fusion pacing therapy to heart 12 of patient 14. For example,processor 80 may detect an atrial pace or sense event (A_(P/S)) (120)and, thereafter, control stimulation generator 84 to deliver a fusionpacing stimulus (LV_(P)) to LV 32 (122), e.g., at a time determinedbased on a fusion pacing interval. In the technique shown in FIG. 13,processor 80 detects the surrogate indication of intrinsic AV conduction(160), e.g., using the techniques shown in FIGS. 6-11 and discussedabove. Processor 80 may, for example, detect a RV 28 sense event(RV_(S)) within a predetermined window of time following the delivery ofthe fusion pacing pulse (LV_(P)) and use the techniques described withrespect to FIGS. 6-11 to determine that the activation of RV 28indicated by the RV sense event was not caused by the delivery of thefusion pacing pulse to LV 32.

Processor 80 determines the A_(P/S)−RV_(S) interval after detecting thesurrogate indication of intrinsic AV conduction (162). In the exampletechnique shown in FIG. 13, processor 80 may also determine theA_(P/S)−RV_(S) interval is attributable to intrinsic conduction, e.g.,using the techniques described with respect to FIGS. 8-11. Afterdetermining the A_(P/S)−RV_(S) interval, processor 80 may adjust afusion pacing interval (A_(P/S)−LV_(P)) based on the A_(P/S)−RV_(S)interval (164). As discussed above, in some examples, processor 80determines a fusion pacing interval based on an intrinsic AV conductiontime measurement. In accordance with the technique shown in FIG. 13,processor 80 may adjust the fusion pacing interval by using theA_(P/S)−RV_(S) interval instead of the intrinsic AV conduction timemeasurement. The A_(P/S)−RV_(S) interval may be, for example, a singleA_(P/S)−RV_(S) interval or a mean, median, greatest, or smallestA_(P/S)−RV_(S) interval of a plurality of A_(P/S)−RV_(S) intervalsdetermined by processor 80 within a predetermined time frame. In someexamples, processor 80 determines the fusion pacing interval based onthe A_(P/S)−RV_(S) interval instead of the intrinsic AV conduction timemeasurement at all times. In other examples, processor 80 onlyperiodically adjusts the fusion pacing interval based on theA_(P/S)−RV_(S) interval instead of the intrinsic AV conduction timemeasurement.

In one example, processor 80 may determine an adjusted fusion pacinginterval (A_(P/S)−LV_(P)) based on the A_(P/S)−RV_(S) interval bydecrementing the A_(P/S)−RV_(S) interval by a PEI, as show in Equation(5) below:

A _(P/S) −LV _(P)=(A _(P/S) −RV _(S))−PEI  Equation (5)

The PEI shown in Equation (5) may be any suitable PEI, such as the onesdescribed above with respect to Equation (1). T

As discussed above, in some examples, processor 80 determines a fusionpacing interval using Equation (1), whereby processor 80 decrements anintrinsic AV conduction time measurement (A_(P/S)−RV_(S, INTRINSIC)) bya PEI in order to determine the fusion pacing interval. In theseexamples, processor 80 may periodically adjust the fusion pacinginterval using Equation (5).

In some examples in which the intrinsic AV conduction time measurement(A_(P/S)−RV_(S, INTRINSIC)) is used to determine (e.g., initiallydetermine or adjust) the fusion pacing interval, the intrinsic AVconduction time measurement (A_(P/S)−RV_(S, INTRINSIC)) is determinedless frequently than the A_(P/S)−RV_(S) interval. For example, processor80 may only determine the intrinsic AV conduction time(A_(P/S)−RV_(S, INTRINSIC)) once every 24 hours, and may determine theA_(P/S)−RV_(S) interval once every cardiac cycle (every heart beat).Thus, processor 80 may be configured to update the fusion pacinginterval based on the A_(P/S)−RV_(S) interval more frequently than basedon the intrinsic AV conduction time measurement. In some examples,processor 80 is configured to adjust the fusion pacing interval on abeat-to-beat basis based on the A_(P/S)−RV_(S) interval determined forthe prior cardiac cycle.

In examples in which processor determines the A_(P/S)−RV_(S) intervalmore frequently than the actual intrinsic conduction time measurement,and processor 80 adjusts a fusion pacing interval based on theA_(P/S)−RV_(S) interval, processor 80 may provide an adjustment to thefusion pacing interval that is more responsive to the cardiac status(e.g., the cardiac output needs) of patient 14 compared to examples inwhich processor 80 adjusts a fusion pacing interval based on actualintrinsic conduction time measurements alone. In addition, or instead,in some examples in which processor 80 is configured to adjust a fusionpacing interval based on the A_(P/S)−RV_(S) interval, processor 80 maydetermine the intrinsic AV conduction time measurement less frequently,such that the suspension of the delivery of pacing stimuli to heart 12that may occur during the intrinsic AV conduction time measurement mayoccur less frequently.

FIG. 14 is a block diagram illustrating a system 170 that includes anexternal device 172, such as a server, and one or more computing devices174A-174N that are coupled to IMD 16 and programmer 24 shown in FIG. 1via a network 176, according to one example. In this example, IMD 16uses its telemetry module 88 (FIG. 3) to communicate with programmer 24via a first wireless connection, and to communicate with an access point178 via a second wireless connection. In the example of FIG. 14, accesspoint 178, programmer 24, external device 172, and computing devices174A-174N are interconnected, and able to communicate with each other,through network 176. In some cases, one or more of access point 178,programmer 24, external device 172, and computing devices 174A-174N maybe coupled to network 176 through one or more wireless connections. IMD16, programmer 24, external device 172, and computing devices 174A-174Nmay each comprise one or more processors, such as one or moremicroprocessors, DSPs, ASICs, FPGAs, programmable logic circuitry, orthe like, that may perform various functions and operations, such asthose described herein.

Access point 178 may comprise a device that connects to network 176 viaany of a variety of connections, such as telephone dial-up, digitalsubscriber line (DSL), or cable modem connections. In other examples,access point 178 may be coupled to network 176 through different formsof connections, including wired or wireless connections. In someexamples, access point 178 may communicate with programmer 24 and/or IMD16. Access point 178 may be co-located with patient 14 (e.g., within thesame room or within the same site as patient 14) or may be remotelylocated from patient 14. For example, access point 178 may be a homemonitor that is located in the patient's home or is portable forcarrying with patient 14.

During operation, IMD 16 may collect, measure, and store various formsof diagnostic data. For example, as described previously, IMD 16 maycollect ECG and/or EGM signals, determine different fusion pacingintervals, and determine different A_(P/S)−RV_(S) intervals. In certaincases, IMD 16 may directly analyze collected diagnostic data andgenerate any corresponding reports or alerts. In some cases, however,IMD 16 may send diagnostic data to programmer 24, access point 178,and/or external device 172, either wirelessly or via access point 178and network 176, for remote processing and analysis.

For example, IMD 16 may send programmer 24 data that indicates whether aloss of intrinsic AV conduction was detected. Programmer 24 may generatereports or alerts after analyzing the data. As another example, IMD 16may send programmer 24, external device 172, or both a plurality ofdetermined A_(P/S)−RV_(S) intervals for, and programmer 24, externaldevice 172, or both, may determine whether A_(P/S)−RV_(S) intervals overtime are increasing, thereby indicating a change in pacing configurationmay be desirable. As another example, IMD 16 may send a system integrityindication generated by processor 80 (FIG. 3) to programmer 24, whichmay take further steps to determine whether there may be a possiblecondition with one or more of leads 18, 20, and 22. For example,programmer 24 may initiate lead impedance tests or IMD 16 may providelead impedance information, if such information is already available.

In another example, IMD 16 may provide external device 172 withcollected EGM data, system integrity indications, and any other relevantphysiological or system data via access point 178 and network 176.External device 172 includes one or more processors 180. In some cases,external device 172 may request such data, and in some cases, IMD 16 mayautomatically or periodically provide such data to external device 172.Upon receipt of the diagnostic data via input/output device 182,external device 172 is capable of analyzing the data and generatingreports or alerts upon determination that there may be a possiblecondition with one or more of leads 18, 20, and 22 or with patient 14.

In one example, external device 172 may comprise a secure storage sitefor information that has been collected from IMD 16 and/or programmer24. In this example, network 176 may comprise an Internet network, andtrained professionals, such as clinicians, may use computing devices174A-174N to securely access stored data on external device 172. Forexample, the trained professionals may need to enter usernames andpasswords to access the stored information on external device 172. Inone embodiment, external device 172 may be a CareLink server provided byMedtronic, Inc., of Minneapolis, Minn.

The techniques described in this disclosure, including those attributedto image IMD 16, programmer 24, or various constituent components, maybe implemented, at least in part, in hardware, software, firmware or anycombination thereof. For example, various aspects of the techniques maybe implemented within one or more processors, including one or moremicroprocessors, DSPs, ASICs, FPGAs, or any other equivalent integratedor discrete logic circuitry, as well as any combinations of suchcomponents, embodied in programmers, such as physician or patientprogrammers, stimulators, image processing devices or other devices. Theterm “processor” or “processing circuitry” may generally refer to any ofthe foregoing logic circuitry, alone or in combination with other logiccircuitry, or any other equivalent circuitry.

Such hardware, software, firmware may be implemented within the samedevice or within separate devices to support the various operations andfunctions described in this disclosure. In addition, any of thedescribed units, modules or components may be implemented together orseparately as discrete but interoperable logic devices. Depiction ofdifferent features as modules or units is intended to highlightdifferent functional aspects and does not necessarily imply that suchmodules or units must be realized by separate hardware or softwarecomponents. Rather, functionality associated with one or more modules orunits may be performed by separate hardware or software components, orintegrated within common or separate hardware or software components.

When implemented in software, the functionality ascribed to the systems,devices and techniques described in this disclosure may be embodied asinstructions on a computer-readable medium such as RAM, ROM, NVRAM,EEPROM, FLASH memory, magnetic data storage media, optical data storagemedia, or the like. The instructions may be executed to support one ormore aspects of the functionality described in this disclosure.

Various examples have been described. These and other examples arewithin the scope of the following claims.

1. A system comprising: an electrical stimulation module configured todeliver cardiac resynchronization pacing therapy to a heart of apatient; a sensing module; and a processor configured to control theelectrical stimulation module to deliver a pacing stimulus to a firstventricle of the heart, and determine whether a surrogate indication ofintrinsic conduction of the heart of the patient is detected after theelectrical stimulation module delivers the pacing stimulus to the firstventricle, the processor being configured to determine whether thesurrogate indication of the intrinsic conduction is detected by at leastdetermining whether the sensing module detected activation of a secondventricle of the heart within a predetermined window of time immediatelyfollowing delivery of the pacing stimulus to the first ventricle by theelectrical stimulation module, and the processor being furtherconfigured to control the cardiac resynchronization therapy delivered bythe electrical stimulation module to the patient based on whether thesurrogate indication of intrinsic conduction is detected after theelectrical stimulation module delivers the pacing stimulus to the firstventricle.
 2. The system of claim 1, wherein the processor is configuredto control the electrical stimulation module to deliver the pacingstimulus to the first ventricle by at least controlling the electricalstimulation module to deliver fusion pacing therapy to the firstventricle, and wherein the processor is configured to control thecardiac resynchronization therapy based on whether the surrogateindication of intrinsic conduction is detected by at least: controllingthe electrical stimulation module to suspend delivery of fusion pacingtherapy to the heart and deliver biventricular pacing therapy to theheart in response to determining the surrogate indication of intrinsicconduction is not detected after the electrical stimulation moduledelivers the pacing stimulus to the first ventricle; and controlling theelectrical stimulation module to continue delivering fusion pacingtherapy to the first ventricle in response to determining the surrogateindication of intrinsic conduction is detected after the electricalstimulation module delivers the pacing stimulus to the first ventricle.3. The system of claim 1, wherein the processor is configured to controlthe electrical stimulation module to deliver the pacing stimulus to thefirst ventricle by at least controlling the electrical stimulationmodule to deliver fusion pacing therapy to the first ventricle, andwherein the processor is configured to control the cardiacresynchronization therapy based on whether the surrogate indication ofintrinsic conduction is detected by at least: in response to determiningthe surrogate indication of intrinsic conduction is not detected afterthe electrical stimulation module delivers the pacing stimulus to thefirst ventricle, controlling the electrical stimulation module tosuspend therapy delivery to the first and second ventricles of theheart, controlling the sensing module to sense electrical activity ofthe heart, and determining a measurement of intrinsic conduction timefrom the atrium of the heart to the second ventricle based on the sensedelectrical cardiac activity; and in response to determining thesurrogate indication of intrinsic conduction is detected after theelectrical stimulation module delivers the pacing stimulus to the firstventricle, controlling the electrical stimulation module to continuedelivering fusion pacing therapy to the first ventricle.
 4. The systemof claim 3, wherein the processor is configured to, in response todetermining the surrogate indication of intrinsic conduction is notdetected after the electrical stimulation module delivers the pacingstimulus to the first ventricle, determine whether the measurement ofintrinsic conduction time is less than or equal to a predeterminedthreshold value, control the electrical stimulation module to continuedelivering fusion pacing therapy to the first ventricle in response todetermining the measurement of intrinsic conduction time is less than orequal to the predetermined threshold value, and control the electricalstimulation module to suspend delivery of fusion pacing therapy to theheart and deliver biventricular pacing therapy to the heart in responseto determining the measurement of intrinsic conduction time is greaterthan the predetermined threshold value.
 5. The system of claim 1,wherein the processor is configured to control the cardiacresynchronization therapy delivered by the electrical stimulation moduleto the patient based on whether the surrogate indication of intrinsicconduction of the second ventricle of the heart of the patient isdetected by at least: incrementing a counter in response to determiningthe sensing module did not detect activation of the second ventriclewithin the predetermined window of time immediately following deliveryof the pacing stimulus to the first ventricle; determining whether avalue of the counter is greater than a predetermined threshold value;controlling the electrical stimulation module to suspend delivery offusion pacing therapy to the heart and deliver biventricular pacingtherapy to the heart in response to determining the value of the counteris greater than or equal to the predetermined threshold value; andcontrolling the electrical stimulation module to continue deliveringfusion pacing therapy to the first ventricle in response to determiningthe value of the counter is not greater than or equal to thepredetermined threshold value.
 6. The system of claim 1, wherein theprocessor is configured to control the cardiac resynchronization therapydelivered by the electrical stimulation module to the patient based onwhether the surrogate indication of intrinsic conduction of the secondventricle of the heart of the patient is detected by at least:incrementing a counter in response to determining activation of thesecond ventricle is not detected within the predetermined window of timeimmediately following delivery of the pacing stimulus to the firstventricle; determining whether a value of the counter is greater than orequal to a predetermined threshold value; suspending therapy delivery tothe heart and determining a measurement of intrinsic conduction timefrom the atrium of the heart to the second ventricle in response todetermining the value of the counter is greater than or equal to thepredetermined threshold value; and controlling the electricalstimulation module to continue delivering fusion pacing therapy to thefirst ventricle in response to determining the value of the counter isnot greater than or equal to the predetermined threshold value.
 7. Thesystem of claim 6, wherein the predetermined threshold value comprises afirst predetermined threshold value, and wherein processor is furtherconfigured to, in response to determining the value of the counter isgreater than or equal to the first predetermined threshold value,determine whether the measurement of intrinsic conduction time is lessthan or equal to a second predetermined threshold value, control theelectrical stimulation module to continue delivering fusion pacingtherapy to the first ventricle in response to determining themeasurement of intrinsic conduction time is less than or equal to thesecond predetermined threshold value, and control the electricalstimulation module to suspend delivery of fusion pacing therapy to theheart and deliver biventricular pacing therapy to the heart in responseto determining the measurement of intrinsic conduction time is not lessthan or equal to the second predetermined threshold value.
 8. (canceled)9. (canceled)
 10. The system of claim 1, wherein the processor isconfigured to control the electrical stimulation module to deliver thepacing stimulus to the first ventricle by at least: detecting a firstatrial pace or sense event; and controlling the electrical stimulationmodule to deliver a first pacing stimulus after expiration of a firstpacing interval from the first atrial pace or sense event, and whereinthe processor is configured to determine whether the surrogateindication of intrinsic conduction from the atrium of the heart to thesecond ventricle of the heart of the patient is detected by at least:detecting, based on electrical cardiac activity sensed by the sensingmodule, a first activation of the second ventricle within thepredetermined window of time immediately following delivery of the firstpacing stimulus to the first ventricle; determining a first timeinterval between the first atrial pace or sense event and the firstactivation of the second ventricle and a second time interval betweenthe first pacing stimulus and the first activation of the secondventricle; detecting a second atrial pace or sense event; controllingthe electrical stimulation module to deliver a second pacing stimulus tothe first ventricle after expiration of a second pacing interval fromthe second atrial pace or sense event, wherein the second pacinginterval is less than the first pacing interval; detecting, based onelectrical cardiac activity sensed by the sensing module, a secondactivation of the second ventricle within the predetermined window oftime immediately following delivery of the second pacing stimulus to thefirst ventricle; determining a third time interval between the secondpacing stimulus and the second activation of the second ventricle;determining whether the first time interval is substantially equal to anintrinsic conduction time measurement; determining the surrogateindication of intrinsic conduction is detected in response todetermining the first time interval is substantially equal to theintrinsic conduction time measurement and the third time interval isgreater than the second time interval; and determining the surrogateindication of intrinsic conduction is not detected in response todetermining the first time interval is substantially equal to theintrinsic conduction time measurement and the third time interval is notgreater than the second time interval.
 11. (canceled)
 12. (canceled) 13.(canceled)
 14. A method comprising: controlling an electricalstimulation module to deliver a pacing stimulus to a first ventricle ofa heart of a patient; after the electrical stimulation module deliversthe pacing stimulus to the first ventricle, determining with aprocessor, whether a surrogate indication of intrinsic conduction of theheart of the patient is detected, wherein determining whether thesurrogate indication of the intrinsic conduction is detected comprisesdetecting activation of a second ventricle of the heart within apredetermined window of time immediately following delivery of thepacing stimulus to the first ventricle; and with the processor,controlling cardiac resynchronization therapy delivered by theelectrical stimulation module to the patient based on whether thesurrogate indication of intrinsic conduction from is detected.
 15. Themethod of claim 14, wherein controlling the electrical stimulationmodule to deliver the pacing stimulus to the first ventricle comprisescontrolling the electrical stimulation module to deliver fusion pacingtherapy to the first ventricle, and wherein controlling the cardiacresynchronization therapy based on whether the surrogate indication ofintrinsic conduction is detected comprises: controlling the electricalstimulation module to suspend delivery of fusion pacing therapy to theheart and deliver biventricular pacing therapy to the heart in responseto determining the surrogate indication of intrinsic conduction is notdetected after the electrical stimulation module delivers the pacingstimulus to the first ventricle; and controlling the electricalstimulation module to continue delivering fusion pacing therapy to thefirst ventricle in response to determining the surrogate indication ofintrinsic conduction is detected after the electrical stimulation moduledelivers the pacing stimulus to the first ventricle.
 16. The method ofclaim 14, wherein controlling the electrical stimulation module todeliver the pacing stimulus to the first ventricle comprises controllingthe electrical stimulation module to deliver fusion pacing therapy tothe first ventricle, and wherein controlling the cardiacresynchronization therapy based on whether the surrogate indication ofintrinsic conduction is detected comprises: controlling the electricalstimulation module to suspend therapy delivery to the first and secondventricles of the heart and determining a measurement of the intrinsicconduction time from the atrium of the heart to the second ventricle inresponse to determining the surrogate indication of intrinsic conductionis not detected after the electrical stimulation module delivers thepacing stimulus to the first ventricle; and controlling the electricalstimulation module to continue delivering fusion pacing therapy to thefirst ventricle in response to determining the surrogate indication ofintrinsic conduction is detected after the electrical stimulation moduledelivers the pacing stimulus to the first ventricle.
 17. The method ofclaim 16, wherein determining whether the surrogate indication of theintrinsic conduction is detected comprises determining the surrogateindication of intrinsic conduction is not detected, the method furthercomprising: determining whether the measurement of intrinsic conductiontime is less than or equal to a predetermined threshold value;controlling the electrical stimulation module to continue deliveringfusion pacing therapy to the first ventricle in response to determiningthe measurement of intrinsic conduction time is less than or equal tothe predetermined threshold value; and controlling the electricalstimulation module to suspend delivery of fusion pacing therapy to theheart and deliver biventricular pacing therapy to the heart in responseto determining the measurement of intrinsic conduction time is greaterthan the predetermined threshold value.
 18. The method of claim 14,wherein controlling cardiac resynchronization therapy delivered by theelectrical stimulation module to the patient based on whether thesurrogate indication of intrinsic conduction of the second ventricle ofthe heart of the patient is detected comprises: incrementing a counterin response to determining activation of the second ventricle is notdetected within the predetermined window of time immediately followingdelivery of the pacing stimulus to the first ventricle; determiningwhether a value of the counter is greater than or equal to apredetermined threshold value; controlling the electrical stimulationmodule to suspend delivery of fusion pacing therapy to the heart anddeliver biventricular pacing therapy to the heart in response todetermining the value of the counter is not greater than or equal to thepredetermined threshold value; and controlling the electricalstimulation module to continue delivering fusion pacing therapy to thefirst ventricle in response to determining the value of the counter isnot greater than or equal to the predetermined threshold value.
 19. Themethod of claim 14, wherein controlling cardiac resynchronizationtherapy delivered by the electrical stimulation module to the patientbased on whether the surrogate indication of intrinsic conduction of thesecond ventricle of the heart of the patient is detected comprises:incrementing a counter in response to determining activation of thesecond ventricle is not detected within the predetermined window of timeimmediately following delivery of the pacing stimulus to the firstventricle; determining whether a value of the counter is greater than orequal to a predetermined threshold value; suspending therapy delivery tothe heart and determining a measurement of intrinsic conduction timefrom the atrium of the heart to the second ventricle in response todetermining the value of the counter is greater than or equal to thepredetermined threshold value; and controlling the electricalstimulation module to continue delivering fusion pacing therapy to thefirst ventricle in response to determining the value of the counter isnot greater than or equal to the predetermined threshold value.
 20. Themethod of claim 19, wherein the predetermined threshold value comprisesa first predetermined threshold value, and wherein the method furthercomprises, in response to determining the value of the counter isgreater than the first predetermined threshold value: determiningwhether the measurement of intrinsic conduction time is less than orequal to a second predetermined threshold value; controlling theelectrical stimulation module to continue delivering fusion pacingtherapy to the first ventricle in response to determining themeasurement of intrinsic conduction time is not less than or equal tothe second predetermined threshold value; and controlling the electricalstimulation module to suspend delivery of fusion pacing therapy to theheart and deliver biventricular pacing therapy to the heart in responseto determining the measurement of intrinsic conduction time is not lessthan or equal to the second predetermined threshold value. 21.(canceled)
 22. (canceled)
 23. The method of claim 14, whereincontrolling the electrical stimulation module to deliver the pacingstimulus to the first ventricle comprises: detecting a first atrial paceor sense event; and controlling the electrical stimulation module todeliver a first pacing stimulus after expiration of a first pacinginterval from the first atrial pace or sense event, and whereindetermining whether the surrogate indication of intrinsic conductionfrom the atrium of the heart to the second ventricle of the heart of thepatient is detected comprises: detecting a first activation of thesecond ventricle within the predetermined window of time immediatelyfollowing delivery of the first pacing stimulus to the first ventricle;determining a first time interval between the first atrial pace or senseevent and the first activation of the second ventricle and a second timeinterval between the first pacing stimulus and the first activation ofthe second ventricle; detecting a second atrial pace or sense event;controlling the electrical stimulation module to deliver a second pacingstimulus to the first ventricle after expiration of a second pacinginterval from the second atrial pace or sense event, wherein the secondpacing interval is less than the first pacing interval; detecting asecond activation of the second ventricle within the predeterminedwindow of time immediately following delivery of the second pacingstimulus to the first ventricle; determining a third time intervalbetween the second pacing stimulus and the second activation of thesecond ventricle; determining whether the first time interval issubstantially equal to an intrinsic conduction time measurement;determining the surrogate indication of intrinsic conduction is detectedin response to determining the first time interval is substantiallyequal to the intrinsic conduction time measurement and the third timeinterval is greater than the second time interval; and determining thesurrogate indication of intrinsic conduction is not detected in responseto determining the first time interval is substantially equal to theintrinsic conduction time measurement and the third time interval is notgreater than the second time interval.
 24. (canceled)
 25. (canceled) 26.(canceled)
 27. A system comprising: means for delivering cardiacresynchronization therapy to a heart of a patient; means for determiningwhether a surrogate indication of intrinsic conduction of the heart isdetected after the means for delivering cardiac resynchronizationtherapy delivers a pacing stimulus to a first ventricle of the heart,wherein the means for determining whether the surrogate indication ofintrinsic conduction is detected comprises means for detectingactivation of a second ventricle of the heart within a predeterminedwindow of time immediately following delivery of the pacing stimulus tothe first ventricle; and means for controlling cardiac resynchronizationtherapy delivered by the means for delivering cardiac resynchronizationtherapy based on whether the surrogate indication of intrinsicconduction is detected.
 28. The system of claim 27, wherein the meansfor delivering cardiac resynchronization therapy is configured todeliver fusion pacing therapy and biventricular pacing therapy to theheart, and wherein the means for controlling the cardiacresynchronization therapy is configured to control the means fordelivering cardiac resynchronization therapy to suspend delivery offusion pacing therapy to the heart and deliver biventricular pacingtherapy to the heart in response to a determination, by the means fordetermining, that the surrogate indication of intrinsic conduction isnot detected after the delivery of the pacing stimulus to the firstventricle, and control the means for delivering cardiacresynchronization therapy to continue delivering fusion pacing therapyto the first ventricle in response to a determination, by the means fordetermining, that the surrogate indication of intrinsic conduction isdetected after the electrical stimulation module delivers the pacingstimulus to the first ventricle.
 29. The system of claim 27, wherein themeans for delivering cardiac resynchronization therapy is configured todeliver fusion pacing therapy and biventricular pacing therapy to theheart, and wherein the means for controlling the cardiacresynchronization therapy is configured to control the means fordelivering cardiac resynchronization therapy to suspend therapy deliveryto the first and second ventricles of the heart and determine ameasurement of intrinsic conduction time from the atrium of the heart tothe second ventricle in response to a determination, by the means fordetermining, that the surrogate indication of intrinsic conduction isnot detected after the electrical stimulation module delivers the pacingstimulus to the first ventricle, and control the means for deliveringcardiac resynchronization therapy to continue delivering fusion pacingtherapy to the first ventricle in response to a determination, by themeans for determining, that the surrogate indication of intrinsicconduction is detected after the electrical stimulation module deliversthe pacing stimulus to the first ventricle.
 30. (canceled)
 31. Anon-transitory computer-readable medium comprising instructions that,when executed by a processor, cause the processor to: control anelectrical stimulation module to deliver a pacing stimulus to a firstventricle of a heart of a patient; after the medical device delivers thepacing stimulus to the first ventricle, determine whether a surrogateindication of intrinsic conduction of the heart of the patient isdetected, wherein the instructions cause the processor to determinewhether the surrogate indication of the intrinsic conduction is detectedby at least detecting activation of a second ventricle of the heartwithin a predetermined window of time immediately following delivery ofthe pacing stimulus to the first ventricle; and control cardiacresynchronization therapy delivered by the electrical stimulation moduleto the patient based on whether the surrogate indication of intrinsicconduction of the heart of the patient is detected.
 32. (canceled)